Adjust PreserveSig emission

[riverar]

Fixes #559

Removes PreserveSig emission for all APIs by default. Adds PreserveSig emission for class methods explicitly marked with [PreserveSig] in emitter.settings.rsp. (Interface methods were already covered previously.)

Metadata diff is too large to be useful, as you can imagine.

[AArnott]

FWIW I feel PreserveSig should remain on standard C methods. Only the COM interface methods should lack preservesig because those are the ones we expect to be projected with a different signature.

[kennykerr]

Hate to disagree but I've been applying transformations on methods and free functions alike and it would be a major step back not to. Developers love it.

[AArnott]

@kennykerr I'm very curious now. Are you saying that win32 methods like CreateFile don't return values anymore? Methods that return hresult, win32 error codes, lstatus, etc... will throw instead of return error codes?

[kennykerr]

It's more nuanced than that. First of all, it's not about "throwing". Rust doesn't have exceptions. It's about permitting or preventing transformation of some kind. Whether it's a method or free function, it can have a signature that may benefit from some transformation. This doesn't mean that every function that lacks the PreserveSig bit will be transformed. The bit just ensures that any transformation that would otherwise apply will be ignored, and the original signature will be preserved.

As an example, CreateFileA will always have its signature preserved and return a handle directly regardless of whether the PreserveSig bit is set because no other transformation is applicable. DoDragDrop on the other hand would be transformed unless the PreserveSig bit is set.

[AArnott]

That makes sense. In CsWin32 we can deal with it either way, so if it's useful to you as-is, that's fine by me.