[core.reflect] TemplateInstance reflection

[Stefan Koch]

Good Evening everyone.

Today I have made progress with reflecting over template 
instances.

TLDR. core.reflect can almost replace `__traits` now and 
preliminary tests show that the reflection data-creation scales 
linearly.
Meaning 10 times as much code takes 10 times as much time to 
reflect.


The following code:
```D
import core.reflect.reflect;

template TypeDef(string name, T)
{
     struct TypeDef
     {
         T value;
         alias value this;
     }
}

alias myInt = TypeDef!("myInt", int);

pragma(msg, () {
     auto n = nodeFromName("myInt", ReflectFlags.Everything);
     string[immutable(ulong)] cmap;
     return nodeToString(n, &cmap);
} ());
```

```JSON
AliasDeclaration = {
     internalPointer: unhandled,
     serial: 599,
     astTypeName: "AliasDeclaration",
     name: "myInt",
     attributes: [],
     linkage: Default,
     comment: "",
     mangledName: "",
     location: Location = {
         filename: "testTemplateReflection.d",
         line: 12,
         column: 1,
     },
     parent: Module = {
         internalPointer: unhandled,
         serial: 580,
         astTypeName: "Module",
         _scope: Scope = {
             internalPointer: unhandled,
             serial: 0,
             astTypeName: "Scope",
         },
         identifier: "testTemplateReflection",
         parent: null,
         members: [
             DeclarationRef to 'myInt',
         ],
         packages: [],
         source_filename: "testTemplateReflection.d",
         string_import_filenames: [],
         declDefs: [],
     },
     storageClasses: [],
     type: TypeStruct = {
         internalPointer: unhandled,
         serial: 31545,
         astTypeName: "TypeStruct",
         kind: "struct",
         alignSize: 4,
         size: 4,
         identifier: "TypeDef!("myInt", int)",
         _unqualified: null,
         sym: StructDeclaration = {
             internalPointer: unhandled,
             serial: 31544,
             astTypeName: "StructDeclaration",
             name: "TypeDef",
             attributes: [],
             linkage: D,
             comment: "",
             mangledName: 
"22testTemplateReflection__T7TypeDefVAyaa5_6d79496e74TiZQBc",
             location: Location = {
                 filename: "testTemplateReflection.d",
                 line: 5,
                 column: 5,
             },
             parent: TemplateInstance = {
                 internalPointer: unhandled,
                 serial: 597,
                 astTypeName: "TemplateInstance",
                 _scope: null,
                 identifier: "",
                 parent: Module = {
                     internalPointer: unhandled,
                     serial: 580,
                     astTypeName: "Module",
                     _scope: Scope = {
                         internalPointer: unhandled,
                         serial: 0,
                         astTypeName: "Scope",
                     },
                     identifier: "testTemplateReflection",
                     parent: null,
                     members: [
                         DeclarationRef to 'myInt',
                     ],
                     packages: [],
                     source_filename: "testTemplateReflection.d",
                     string_import_filenames: [],
                     declDefs: [],
                 },
                 members: [
                     DeclarationRef to 'TypeDef',
                 ],
                 name: "TypeDef",
                 decl: TemplateDeclaration = {
                     internalPointer: unhandled,
                     serial: 594,
                     astTypeName: "TemplateDeclaration",
                     name: "TypeDef",
                     attributes: [],
                     linkage: Default,
                     comment: "",
                     mangledName: 
"22testTemplateReflection7TypeDef",
                     location: Location = {
                         filename: "testTemplateReflection.d",
                         line: 3,
                         column: 1,
                     },
                     parent: Module = {
                         internalPointer: unhandled,
                         serial: 580,
                         astTypeName: "Module",
                         _scope: Scope = {
                             internalPointer: unhandled,
                             serial: 0,
                             astTypeName: "Scope",
                         },
                         identifier: "testTemplateReflection",
                         parent: null,
                         members: [
                             DeclarationRef to 'myInt',
                         ],
                         packages: [],
                         source_filename: 
"testTemplateReflection.d",
                         string_import_filenames: [],
                         declDefs: [],
                     },
                     storageClasses: [],
                     isEponymous: true,
                     constraint: null,
                 },
             },
             storageClasses: [],
             type: TypeRef to 'TypeDef!("myInt", int)',
             fields: [
                 VariableDeclaration = {
                     internalPointer: unhandled,
                     serial: 31547,
                     astTypeName: "VarDeclaration",
                     name: "value",
                     attributes: [],
                     linkage: D,
                     comment: "",
                     mangledName: 
"_D22testTemplateReflection__T7TypeDefVAyaa5_6d79496e74TiZQBc5valuei",
                     location: Location = {
                         filename: "testTemplateReflection.d",
                         line: 7,
                         column: 11,
                     },
                     parent: DeclarationRef to 'TypeDef',
                     storageClasses: [
                         Field,
                         Ctorinit,
                     ],
                     type: TypeBasic = {
                         internalPointer: unhandled,
                         serial: 138,
                         astTypeName: "TypeBasic",
                         kind: "int",
                         alignSize: 4,
                         size: 4,
                         identifier: "int",
                         _unqualified: null,
                     },
                     _init: null,
                     offset: unhandled,
                 },
             ],
             members: [
                 DeclarationRef to 'value',
             ],
         },
     },
     decl: null,
}

```

As you can see it is possible to look through the type alias 
myInt Into the template-instance TypeDef!("myInt", int) and 
through there into the eponymous struct Declaration.

I am aware that the produced tree is fairly big however I've 
thought that a more faithful representation of the Tree inside 
the compiler would be useful to more people than a cut down 
version which is tailored to specific use-cases that I know of.

As always I would be very happy about positive feedback or 
constructive criticism.

Have a very nice day/night/evening/morning.

Cheers,
Stefan