Convert `struct FromBytesWithNulError` into enum

One of `CStr` constructors, `CStr::from_bytes_with_nul(bytes: &[u8])` handles 3 cases:
1. `bytes` has one NULL as the last value - creates CStr
2. `bytes` has no NULL - error
3. `bytes` has a NULL in some other position - error

The 3rd case is error that may require lossy conversion, but the 2nd case can easily be handled by the user code. Unfortunately, this function returns an opaque `FromBytesWithNulError` error in both 2nd and 3rd case, so the user cannot detect just the 2nd case - having to re-implement the entire function and bring in the `memchr` dependency.

In [this code](https://github.com/gquintard/varnish-rs/blob/f86d7a87683b08d2e634d63e77d9dc1d24ed4a13/varnish-sys/src/vcl/ws.rs#L158), my FFI code needs to copy user's `&[u8]` into a C-allocated memory blob in a NUL-terminated `CStr` format.  My code must first validate if `&[u8]` has a trailing NUL (case 1), no NUL (adds one on the fly - case 2), or NUL in the middle (3rd case - error). I had to re-implement `from_bytes_with_nul` and add `memchr`dependency just to handle the 2nd case.

This PR renames the former `kind` enum from `FromBytesWithNulErrorKind` to `FromBytesWithNulError`, and removes the original struct.

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