We reorganize jsCoq's source code as to be more modular, in particular
we start to place backend files under the `backend` directoy, and
frontend files under `frontend`.

Code as it now does work, note however that still quite a bit of work
is needed. In particular, the headless frontend should get its own
directory, the coq-jslib directory should go away, use more types,
etc...

To avoid confusion, `ui-js/jscoq.js` has been renamed.
Also, a legacy-mode `jscoq-loader.js` that emulates the old behavior.

We use a strategy top-down to convert to modules. For now we convert
the loader to a module. This requires a few minor changes, but
shouldn't be too hard.

Coq standard library is now bundled in the `'coq'` package bundle.

Fixes #210.

I have done a quick pass and tried to tidy the readme up a bit.

Please don't hesitate to further refine.