Do not support non-UTF-8 bases any longer

Got rid of the global reference utf_8_db and some dead code.
Update deprecated functions
Array.create -> Array.make
Lazy.lazy_from_fun -> Lazy.from_fun
Preparing for safe_strings
configure: check ocaml version
Force safe-string (requires OCaml 4.02+)
Conversion to -safe-string, first steps
configure: ocaml 4.02 needed
Compile with opam and OCaml 4.02