Support jsdoc output larger than available RAM

[erikrose]

Incrementally read the JSON using something like https://pypi.python.org/pypi/ijson/ (which looks a little unusual (strange docstring formatting, needless pass use, implements its own hand-written JSON parser) but might be okay). Then keep just the indices (app._sphinxjs_doclets_by_path and app._sphinxjs_doclets_by_class) in RAM, and push the rest out to disk. In any case, we'll want to stop storing the doclets themselves in the indices and replace them with some sort of references.

Crazy brainstorming:

• Replace each doclet with just begin/end byte offsets where they occur in the jsdoc-emitted file, and read them in as needed, reparsing each time. Pro: no need to write all the doclets twice.