Little improves in CString `new` when creating from slice

Old code already contain optimization for cases with `&str` and `&[u8]` args. This commit adds a specialization for `&mut[u8]` too.

Also, I added usage of old slice in search for zero bytes instead of new buffer because it produce better code for Windows on LTO builds. For other platforms, this wouldn't cause any difference because it calls `libc` anyway.

Inlined `_new` method into spec trait to reduce amount of code generated to `CString::new` callers.

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