opam-street: 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.

See seliopou/opam-street to see how the picked opam is created.
Merge pull request #241 from frenetic-lang/opam-street

use cached .opam