One step out of the TypeInfo stalemate

[Andrei Alexandrescu]

I think this has been attempted before. The TypeInfo* classes in 
object.d are one gnarly thing after another:

https://github.com/dlang/druntime/blob/master/src/object.d

Very poor usefulness to real estate ratio, ugly and inefficient 
implementation, etc. etc.

One simple step out of this would be to replace TypeInfo_Struct with a 
template TypeInfo(T) if (is(T == struct)). Then expressions such as 
`typeid(MyStruct)` would be lowered to 
`.object.__getTypeInfo!MyStruct()`. That function uses a singleton with 
lazy allocation.

That means for all structs the compiler does not need to generate 
TypeInfo_Struct objects, and the implementation of the primitives is 
simpler and more efficient.

We can't do exactly this for classes because typeid(obj) is dynamic, so 
it requires a virtual call. But there are other ideas we can use there 
as well.