Define rust::isize with Windows support

[dtolnay]

Hybrid of 15b5b10 and ef1bf18 and #96 (comment).

cc @myronahn @AlesTsurko
closes #96

[dtolnay]

Following up on _WIN32 vs _MSC_VER from #96 (comment): it sounds like _WIN32 is defined when building for windows with any compiler, while _MSC_VER is defined when building specifically with the Microsoft compiler. Unclear which is more appropriate in this case but I would think _WIN32 because building the same code with a different compiler on windows isn't going to cause a different set of headers to be included (is it?). @myronahn @AlesTsurko I would appreciate any insight on this.

[ales-tsurko]

Really can't say anything reasonable about it. I'm not an expert in the Windows development.

Anyway, if this is true:

_WIN32 is defined when building for windows with any compiler, while _MSV_VER is defined when building specifically with the Microsoft compiler.

the _WIN32 is the correct choice.

I used the Visual Studio's/Microsoft compiler to check #96

[myronahn]

I believe BaseTsd.h is in the Windows SDK and so theoretically independent of the Microsoft Compiler. _WIN32 is probably more appropriate in this case.

[dtolnay]

Thanks all, I'll go ahead with this approach then. It looks like both our x86_64-pc-windows-msvc and x86_64-pc-windows-gnu CI builds are happy with this which is a good sign. Please let me know if this solution turns out to be problematic down the line.