`ProbablyPortable`: A potential protector against platform peculiarities

[jswrenn]

I propose a safe, derivable ProbablyPortable trait, which provides a (very good) best-effort analysis and marker of probable portability. If a type is ProbablyPortable it probably:

• has consistent layout across compilations, platforms and toolchains
• does not have an endian-sensitive representation

We will provide base implementations of ProbablyPortable for (), bool, i8, u8, and zerocopy's endian-aware types. An implementation is also provided for arrays of ProbablyPortable elements.

derive(ProbablyPortable) can be applied to:

• structs and unions marked repr(C) or repr(transparent), whose members are ProbablyPortable
• enums with an explicit integer repr, whose members are ProbablyPortable

The documentation of ProbablyPortable will instruct users that it should not be derived on types whose APIs are endian aware in non-portable ways. However, it is not, strictly speaking, a violation of ProbablyPortable's contract to do so — the proposed trait is called ProbablyPortable, not Portable, because a derive cannot guarantee this.