Bundle coq

This commit series bundles HoTT/coq via git submodules.  This simplifies the build process in some ways, and mostly makes the source directory readonly (though I haven't tested that), but it unfortunately ties the library somewhat strongly to using git.

I also use bash arrays in `configure`, which isn't the most portable, but I cannot think of a better way to allow customization of the bundled Coq.

The new install procedure is "clone HoTT/HoTT, then run `./autogen.sh && ./configure && make` and then start contributing (assuming you have OCaml and all of the Coq dependencies).