RAM usage: find a way to memory map the English lexicon

[joanise]

g2p is used on the ReadAlong Studio back-end server, and causes a fairly high amount of RAM to get used.

• importing networkx (any subpart of it, even just the top-level import) costs about 30MB (see RAM usage: replace networkx elements used by the web API server #394)
• importing the languages database costs about 60MB, 40 or 50 of which is likely the English lexicon

This is more complex than replacing networkx algorithms: if we could find a way to memory map the English lexicon from disk, instead of loading in into RAM, that would let us realize a significant memory saving on the API server, for both the g2p backend and the RAS web-api back-end.