Proposal: change regexp backend

[leque]

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.

[puripuri2100]

The regexp engine implemented by the SATySFi library has the disadvantage of "Slow".

[leque]

After some investigation, I realized that ocaml-re does not support Unicode (ocaml/ocaml-re#24). So pcre-ocaml + `UTF8 flag would be a preferred choice. Unicode support is also required to fix a problem that regexp primitives (string-scan (regexp `.`) `あいう` etc.) may generate a invalid string (cf. #56).