Utilize function pointers for interop

[JeremyKuhne]

Function pointers provide better performance. Creating this issue to discuss utilizing them in Windows Forms interop.

Things we should consider / look at:

• Win32 callbacks
• High usage COM interfaces
• ?

[JeremyKuhne]

Some Runtime changes:

dotnet/runtime#43514
dotnet/runtime#39752
dotnet/runtime#39082

[elachlan]

@RussKie Should we close this issue because of #7445 or does this compliment it?

[RussKie]

Thank you for going through the backlog. If we're importing callbacks from CsWin32 then, I think, we can close it. If we still have manually created callbacks - then I think this may still be relevant.

I let @JeremyKuhne to make a call on this, since he's opened the issue.

[JeremyKuhne]

It is related to the other, but we should keep it open to attach known instances of this.

[JeremyKuhne]

We've been doing this now, no longer a need to track.