ZArith v1.5

• Install all .cmx files, improving performance of clients and avoiding a warning from OCaml 4.03 and up.
• Z.of_float: fix a bug in the fast path [Richard Jones] (See https://bugzilla.redhat.com/show_bug.cgi?id=1392247)
• Improve compatibility with OCaml 4.03 and up [Bernhard Schommer]
• Overflow issue in Z.pow and Z.root with very large exponents (GPR#5) [Andre Maroneze]
• Added function Q.to_float.