Implement TryInto<CString> for str and String

[kornelski]

Higher-level FFI wrappers that need to present a CString to C don't have a convenient + efficient type to take on the Rust side.

ffi_string(arg: ???) {
   let c_string = CString::new(arg)?;
   ffify(c_string.as_ptr());
}

• Taking CString is cumbersome, as it doesn't work with simple &str and requires callers to do the conversion and handle NUL errors.

• Taking String or Vec<u8> is efficient only if it happens to have spare capacity, but "foo".to_string() doesn't have any spare capacity.

• Taking &str always needs to allocate.

Taking impl TryInto<CString> would be ideal, if that trait was implemented for common string-like types.

[EvanCarroll]

There is already .into_string() I would love to know why this isn't deprecated and TryInto implemented instead.

[kornelski]

cstring.into_string is a conversion in the opposite direction than what I had in mind, but I wouldn't mind if both were supported via TryFrom/TryInto.