CStr from fixed c_char array

Hi,

Often times I see a fixed array of C chars returned via FFI boundary which cannot be easily converted into CStr because:

1. CStr::from_ptr() uses strlen() which can walk outside of boundaries of a fixed array
2. CStr::from_bytes_until_nul(&[u8]) takes u8 slice.

Each time I stumble upon [c_char; N] it's always a struggle.

As such I think CStr should have an initializer from &[c_char] slice. This would work identical to CStr::from_ptr() except it wouldn't need to use strlen. It could flip the slice into &[u8] and then pass it into from_bytes_until_nul.

Start by reviewing the existing CStr::from_ptr and CStr::from_bytes_until_nul entry points described in the issue, along with how [c_char] slices cross the FFI boundary. Define the bounded conversion's API and safety behavior, then add coverage showing fixed c_char arrays are handled without scanning beyond the slice.