Support compiling in Windows \\?\ verbatim location

[dtolnay]

This PR implements a workaround for rust-lang/cargo#13919.

See rust-lang/cargo#13919 (comment) and rust-lang/cargo#13919 (comment) of 2 different users reporting experiencing build errors in typenum.

error: couldn't read \\?\C:\myproj\rust\gui\gpuimg\target\debug\build\typenum-8c302cc9c705de15\out/op.rs: 文件名、目录名或卷标语法不正确。 (os error 123)
  --> C:\rust\cargo\registry\src\rsproxy.cn-0dccff568467c15b\typenum-1.17.0\src\lib.rs:72:5
   |
72 |     include!(concat!(env!("OUT_DIR"), "/op.rs"));
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

build\typenum-255e67d6ee89f566\out/op.rs: The filename, directory name, or volume label syntax is incorrect. (os error 123)
  --> src\lib.rs:72:5
   |
72 |     include!(concat!(env!("OUT_DIR"), "/op.rs"));
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: this error originates in the macro `include` (in Nightly builds, run with -Z macro-backtrace for more info)

Rustc 1.84 contains a mitigation for this (rust-lang/rust#125205) but is very recent (5 weeks old) compared to typenum's oldest supported compiler which is 1.37.

[paholg]

Supporting these paths on various Windows setups has been quite the saga. It seems every time we fix it someone, it breaks for someone else.

It also looks like I need to address CI failing.

I think I'm at the point where it makes most sense to just generate these files locally and check them in; I'm not sure why I didn't start doing that long ago.

Thanks for the PR; let's keep it open for now, and merge if I don't end up getting to my alternate approach, though I plan to do that tonight.

[paholg]

Fixed in #219.