[nbdd0121]

CStr is overhauled to make using it more similar to use a str.

This is split from #258, without procedural macros and CBoundedStr and does not depend on build_assert!.

Some additional improvements:

• Added a BStr which is just [u8] but provide an extra semantical annotation;
• Make CStr deref toBStr so many functions could be used;
• Add AsRef<BStr> for CStr, combining with deref this allows CStr to be used when &BStr is exepcted;
• Allow CStr to be sliced like a [u8];
• Change c_string_bytes to CStr::from_char_ptr

r? @ojeda @wedsonaf