Lexer refactor

[rtfeldman]

Refactored lexer in support of removing let (which is almost there; 1 failing test cases remaining due to where).

The main changes are:

1. Put the keywords in a map, so we can easily check if something is a keyword or not by checking its presence in the map.
2. Parameterize the tokenize functions on chunk, so we can process a substring of chunk instead of the whole thing.
3. Pull out a tokenise function that can be used outside the main exports.tokenise.

This is a big enough change to a critical piece of code that I figured a PR would be appropriate.

[rtfeldman]

Any objections to merging this in?

[joneshf]

I'm game.