Hi Guix!

This patch adds the OCaml bindings for the z3 package. I install it to
a separate output (but this only saves 3MB, maybe not that important).
I'm thinking we could build other bindings too, and it would be nice to
separate them also (after all, we probably don't need all of them at
the same time :)).

One issue is that this package installs the bindings to the Z3 ocaml
package. However, in opam, they override the install directory to be
z3, so we might have issues with dependents. Do you think I should also
override the name of the directory?