Proposal: change regexp backend #306

Currently, SATySFi uses OCaml's Str module as the regexp backend. But it has some drawbacks:

• It supports fewer syntax.
• It has global states (it will be fixed in OCaml 4.14, though).

So, how about changing the regexp backend from Str to, say, ocaml-re? It supports large subsets of PCRE, has nice performance, and has no global states. Or, pcre-ocaml? It might be another choice not to have regexp primitives in the core, and use a library-implemented regexp engine, such as satysfi-base's.