Remove rayon dependency

[kennykerr]

This is only used to speed up package generation, which is mostly just for the generation of the windows and windows-sys crates, by parallelizing code generation. Unfortunately, it has proven unreliable on Windows. I'll likely provide a simple parallelizing replacement in the near future.

[ChrisDenton]

I'm all for simplifying the dependency tree but was there a particular issue motivating this?

[kennykerr]

was there a particular issue motivating this

Running cargo run -p tool_bindings --release hangs my entire machine pretty regularly. At least once or twice a day I'm forced to hard reboot my dev box... This does not happen with my C++ code generators, which hammer the machine at least as hard.

[MRahilly]

Possibly like the file system churn that C++/WinRT implementation headers generate on an older PC.