Micro Optimization - Use ES2015 Maps for Lookahead function Caching.

[bd82]

Accessing the dictionary for the lookahead functions cache is a hotspot.
For example a past optimization of creating the lookahead keys using bit operators
yielded a 25% overall performance boost.

Perhaps using ES6 Maps could provide a performance boost
As they have in Jest:

• HasteMap with native maps jestjs/jest#6960

[bd82]

Looks like there is a smaller performance boost (6-9%) for the parsing flow (excluding lexing flow)
in the more complex grammar samples, With a similar regression for a very simple grammar (JSON).

This is probably a worthwhile trade off.