ZArith v1.9

• Issue #50: add opam file, make it easy to "opam publish" new versions
• Issue #38: configure detects 32bit OCaml switch on 64bit host
• Fix #36: change Q.equal, leq, geq comparisons for undef
• Request #47: move infix comparison operators of Z in submodule,
  avoid shadowing the polymorphic compare
• Fix #49: INT_MAX undeclared
• Request #46: add prefixnonocaml option ()
• Request #45: fix ocamllibdir/caml/mlvalues.h bug (Cygwin)
• Fix: attempting to build numbers too large for GMP raises an OCaml exception
  instead of crashing with "gmp: overflow in mpz type"