Idea: Opaque, repr(transparent) C types

Essentially, replace the types in os::raw with new versions in core::ffi which are opaque, repr(transparent) types. This allows us to add, e.g. a c_longdouble which does not necessarily map to a native Rust type.

Conversion to/from these types should be done using From and TryFrom, where From is only implemented strictly according to the C spec (e.g. From<u32> is not implemented for c_uint, but From<u16> is). For floats, conversions would probably not be done by From and TryFrom, but that can be fleshed out later; adding just the integer types for now would be very nice.

Not 100% sure whether these types should implement basic arithmetic operations or methods representing functions from C math, but that's my basic idea right now.

c_void is already in core::ffi, so, these types would be joining it.

I've had ideas for drafting an RFC for this but I doubt I'll have the energy any time soon, so, I figured I'd offer the idea here if anyone wants to take it up.

Start by reviewing the existing types in os::raw and core::ffi::c_void. Work through the proposed opaque repr(transparent) integer types, their From and TryFrom rules, and the unresolved arithmetic and floating-point questions. Done means the design is sufficiently settled for an RFC or implementation plan.