language model efficient top_k

[smassung]

(as referenced in #107)

[smassung]

See if we can store context -> (id, prob), (id, prob), ... in the forward index format.

[skystrife]

You'd need a map from context -> id though, which is probably too expensive. The postings format requires contiguous ids, too, so just saying id = hash(context) isn't sufficient.