The string schism

[smondet]

• finally do the c_string Vs byte_array schism (fixes #12)
• switch to jbuilder
• revamp the custom testing framework

[smondet]

@leo the necessity of the split is in the issue #12 and the talk and it all comes down to this piece of C code: http://wr.mondet.org/slides/OCaml2017-Genspio/#/c-strings-vs-byte-arrays