Cxx should suggest using i8 or u8 when signed/unsigned char type is attempted

[Xaeroxe]

Maybe only char is needed? I guess signed char and unsigned char will always be the same as i8 and u8 on platforms that Rust supports.

Originally posted by @dtolnay in #120 (comment)

That's all well and good, though it'd be better if the cxx::bridge macro told you to use i8 or u8 instead of saying the signed/unsigned variant of char isn't supported.