Missing conversion to/from `str` with the nul terminator

[kupiakos]

I expect these APIs to be available but they are not:

• fn as_str_with_nul(&self) -> &str
• fn from_str_until_nul(v: &str) -> Result<&CStr8, core::ffi::c_str::FromBytesWithNulError>
• fn from_str_with_nul(v: &str) -> Result<&CStr8, core::ffi::c_str::FromBytesWithNulError>
• unsafe fn from_str_with_nul_unchecked(v: &str) -> &CStr8

These can be worked around with unnecessary runtime checks.