add From<bytes> for StdString

[ctaggart]

See if we can simplify

let name = StdString::from_bytes_with_nul(b"helloworld\0");

to

let name: StdString = b"helloworld\0";

Also use CString to wrap a regular Rust string.

[ctaggart]

I couldn't get this to work with &[u8]. It complains about no implementation for &[u8;24] or similar. I think it needs constant generics. https://github.com/rust-lang/rust/issues/44580

I implemented it for &str.