We could deprecate c_uchar and c_schar

[gnzlbg]

We guarantee that c_uchar and c_schar are always equal to u8 and i8 and we don't aim to support any platforms where this isn't the case, ever. So we could deprecate these and just recommend users to use u8 and i8 instead.

[maxbla]

As a small counterpoint, from a user's perspective, using c_uchar and c_schar can indicate that FFI is going on.

[kellda]

This would be consistent with the deprecation of {u,}int{8,16,32,64}_t in #1304

[tgross35]

These are defined in core::ffi now. Since they are defined there, I don't think there is any point in removing anything here.