Implement startup code injector

[jkotas]

The manually managed startup code needs to be replaced with real managed one.

#468

[MichalStrehovsky]

Before we start working on this, do we want to make GC static section and string literals lazily initialized?

(To keep the stuff the startup code needs to run minimal.)

[MichalStrehovsky]

If this is tracking what the .NET Native for UWP startup code injector does, I think the only thing that's left is static constructors on the <Module> type (nice to have, but we could live without it for a while).

Or is this tracking complete removal of the C++ bootstrapper code?

[jkotas]

I agree - we should have more specific issues on what is left. (#657 tracks the module constructors.)