0.18

This releases marks the addition of QCheck2, a module where generation
and shrinking are better integrated.
See #109 and #116.

This API is still experimental. The normal QCheck module is still there
and hasn't changed much.

deprecations and breakges:

• make QCheck.Test_result.t abstract and add missing getters
• deprecate QCheck.oneof
• deprecate Gen.string_readable in favor of Gen.(string_of char) or the new Gen.string_printable
• require at least OCaml 4.08

other changes:

• unsigned int32 and int64
• rename small_int_corners
• add ?ratio to opt, to modify random distribution of options