Interoperation with C unions

[brson]

Rust only has safe unions, so interoperating with C unions is ugly. One approach to this 'unsafe enums' is proposed in #724.

[huonw]

Relevant discussion:

• FFI and union rust#5492
• https://internals.rust-lang.org/t/pre-rfc-unsafe-enums-now-including-a-poll/2873

[joshtriplett]

RFC pull request: #1444

[alexcrichton]

cc #371

[Stebalien]

Triage: fixed by #1444