Turn CStr into a DST
Turn CStr into a DST

As decided in process of developing the CStr RFC [1], this is the way to go
with CStr as an unsized type.

The length of the internal slice is bogus; it only provides safe access to
the first byte for the implementation of .is_empty().

[1] https://github.com/rust-lang/rfcs/pull/592

mzabaluev/rust-c-str