use cached .opam

Rather than rebuilding the Jane Street libraries and cstruct for each build, this commit will use a pickled .opam directory containing those prebuilt libraries and copy that in place.

This has been tested using [seliopou/frenetic][frenetic] ([travis-ci build][frenetic-ci]). See [seliopou/opam-street][opam-street] to see how the picked opam is created.

[frenetic]: https://github.com/seliopou/frenetic
[frenetic-ci]: https://travis-ci.org/seliopou/frenetic/builds/18110480
[opam-street]: https://github.com/seliopou/opam-street