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

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.

Start by locating the CStr8 implementation and any existing string-conversion tests; the issue names the four missing APIs: as_str_with_nul, from_str_until_nul, from_str_with_nul, and from_str_with_nul_unchecked. Done means these conversions are available with the listed Result and unsafe signatures, with tests covering their expected behavior.

Written by the indexing model from the issue text.