`TryFromBytes`/`IntoBytes` for `CStr`

[kupiakos]

core::ffi::CStr doesn't implement TryFromBytes or IntoBytes, but it meets the requirements for both.

Currently core::ffi::CStr is a wrapper around [u8], but theoretically it'll change so &CStr is a thin pointer.

Before this lands, we should seriously consider:

• Is <CStr as TryFromBytes>::try_ref_from_bytes and <CStr as IntoBytes>::as_bytes eventually having dynamic performance unacceptable?
• Is it risky to use mem::size_of_val for a CStr? mem::size_of_val(&cstr) currently works today, so it'd be very surprising if that breaks for CStr even when extern types land. Would upstream really break its current behavior on stable?
• zerocopy would always include the nul byte in as_bytes - could that be confusing?