v4.0.0

CHANGES:

• sexp: use the sexplib v0.13 ppx directives (@avsm, #143).
• rework the URI parser with angstrom (@anmonteiro, review @avsm & @dinosaure, #142).
• add simple fuzzer tests between angstrom parser and legacy parser (with re.posix, #142)
• add support of modifying pct encoding (with a custom one) (@orbitz, review @anmonteiro, @tmcgilchrist, @avsm & @dinosaure, #147)
• allow the selection of generic set of safe characters (with Generic) (@madroach, review @dinosaure, #141)