Runtime: use monomorphic representation for MlBytes

[hhugo]

Using deoptigate to check performances/optimizations

probably blocked by #1045

[hhugo]

If we make Bytes.unsafe_of_string O(n), we would need to fix String.sub, String.get_intN_*, String.get_uintN_*, String.get_utf_*, String.is_valid_utf_*. They convert the whole string argument to bytes but only look at a small subset.