Pre-RFC: Make `*CStr` a Thin Pointer

[mzabaluev]

@kennytm

I think that it is better to introduce CString::push_c_str() and similar methods (will need another RFC), instead of making it equivalent to Box<CStr>.

During discussions around the time CStr was introduced, the prevailing idea was that CString should not become a mutable container in parallel to String/Vec, but be convertible to and from these.