Add CString cont.

[raviqqe]

This PR is a continued work of #342, the heapless implementation of alloc::ffi::CString. Credits to @vrmiguel.

Closes #330.

Notable changes from the previous PR

• Implement Deref<Target = CStr> for CString.
  • As well as Default, AsRef, Borrow, PartialEq, Eq, PartialOrd, and Ord
• Remove the len and as_str_unchecked associated functions that should be implemented in the upstream of core::ffi::CStr.
• Rename the push_bytes associated function extend_from_bytes.
  • This change is to align the nominology with heapless::Vec.

Future work

• heapless::String interoperability
  • CString::into_string
• heapless::Vec interoperability
  • CString::from_vec_with_nul
  • CString::into_bytes
  • CString::into_bytes_with_nul

[raviqqe]

Should we rename this extend_from_bytes or extend_from_slice?

[raviqqe]

I renamed it extend_from_bytes for now. extend_from_slice is a bit ambiguous because it might mean either byte or CStr slices.

[vrmiguel]

I was wondering if we should use CStr::from_bytes_with_nul here directly (it uses the memchr crate)

    match CStr::from_bytes_with_nul(bytes) {
        Ok(_cstr) => {
            unsafe { self.extend_from_bytes_unchecked(bytes) }?;
            Ok(())
        },
        Err(FromBytesWithNulError::InteriorNul { position }) => {
            Err(ExtendError::InteriorNulByte(position))
        },
        Err(FromBytesWithNulError::NotNulTerminated) => {
            unsafe {
                self.extend_from_bytes_unchecked(bytes).unwrap();
                self.inner.push_unchecked(0);
            };
            Ok(())
        }
    }
}

[reitermarkus]

Makes sense.

[vrmiguel]

Awesome stuff @raviqqe !