Upstream mach definitions in heim-common

[cjbassi]

https://github.com/heim-rs/heim/blob/master/heim-common/src/sys/macos/mod.rs

Should these definitions be upstreamed?

[svartalf]

Yeah, it would be nice to :) There are few extra things in the sources to contribute into winapi and mach crates; #62, for example, was done already.

Yet, it is an extremely low priority right now for me, so if you are up to do this, I'll be glad to help with anything I can.