Rescript support

Some preliminary work has done in this commit: https://github.com/linoscope/CAMLBOY/commit/5adaa2b8a3c5109ee844b19a7ca886fe7346e022

But if I remember correctly, I was blocked by Bigstringaf not supporting rescript, which is because rescript doesn't support bigarray.

Refs:
- https://github.com/inhabitedtype/angstrom/issues/142
- https://github.com/rescript-lang/rescript-compiler/issues/115