OCaml 4.02 introduced the 'bytes' type, a mutable string intended to
replace the existing 'string' type for those cases where the byte
array can be mutated. In future the 'string' type will become
immutable. This is not the default now, but it can be forced using
the '-safe-string' compile option.
This commit changes the code so that it could be compiled using
'-safe-string' (but does not actually make that change).
If we detect OCaml < 4.02, we create a dummy 'Bytes' compatibility
module ((nearly) an alias for the 'String' module). The only
significant difference from upstream OCaml is that you must write the
'bytes' type as 'Bytes.t' in interfaces, apart from that everything
else should work.
6e79a3c84d