Shared library support: Where we are at

[Rikki Cattermole]

Hello!

Currently, I'm doing a push towards getting shared library 
support in D into a much more usable state.

My approach has been this:

Build -betterC shared library, build full D executable that 
depends upon said shared library. Regardless of how you feel 
about -betterC, the reason it is in use here is to try and 
prevent issues surrounding druntime during my initial push.

I have not tested on Posix or used GDC. Right now I am only 
interested in Windows support as it has the most 
platform-specific issues and I am entirely ignoring Optlink (as 
far as I'm concerned that is WONTFIX).

Both ldc and dmd can work in this arrangement. However in both 
cases you need to manually generate ModuleInfo stubs. Like this:

```d
export extern(C) void _D6dynlib3app12__ModuleInfoZ() {
     asm {
         naked;
         di 0;
         di 0x1000;
         db 0;
     }
}
```

Both LDC and DMD do not have a way to configure ModuleInfo 
generation. GDC does have a way to turn it on and off. LDC bug 
report [0].

If you remove ``-betterC``, that stub will no longer work in dmd 
due to duplicate symbols (that wasn't exported). LDC will compile 
okay.

In my codebase I have found that ``--fvisibility=public`` is a 
very useful crutch given export is a linker directive that is 
pretending to be a visibility modifier (however let's ignore that 
since that is a DIP).

As part of my codebase I'm working towards getting dub working. 
I'm currently waiting on a PR that is blocked by a vibe.d 
regression with openssl[1]. This should mean build artifacts from 
dependencies will be copied to the target directory of a project 
and DLLs won't be linked against but instead will use their 
import libraries.

I've cherry-picked a few issues surrounding this [2], [3], [4].

However here are the things that if fixed, should lead to this 
working:

- In both LDC and DMD, implement switches to turn on and off 
ModuleInfo generation (ignoring -betterC switch).
- In DMD export ModuleInfo.
- In DMD implement a switch to turn on export for all symbols.

So to cap this off: right now DMD is basically not even an option 
for shared libraries unless you use -betterC or treat it as if it 
was a non-D library. LDC is possible for non ``-betterC``.

[0] https://github.com/ldc-developers/ldc/issues/3996
[1] https://github.com/dlang/dub/pull/2259
[2] https://issues.dlang.org/show_bug.cgi?id=22367
[3] https://issues.dlang.org/show_bug.cgi?id=9816
[4] https://issues.dlang.org/show_bug.cgi?id=4071