Performance tweaks for TokenSet#intersect.
Performance tweaks for TokenSet#intersect.

More changes related to objects entering 'hash' mode and defeating V8s
optimisations. Iterating over these hash like objects is more performant
when using Object.keys and a for loop rather than the more vanilla
for-in loop.

olivernn/lunr.js,olivernn/lunr.js,olivernn/lunr.js