Allow multi-char keys

[gabejohnson]

Motivation:

The sweet.js table-driven reader uses simple tries to do determine value or the value property on KeywordTokens and PunctuatorTokens. It's fast and it works, but it would be more user-friendly to allow:

const tEntries = [{
  key: 'this',
  action(stream) {
    return KeywordToken({ value: stream.readString(4); });
  }
}, {
  key: 'throw',
  action(stream) {
    return KeywordToken({ value: stream.readString(5); });
  }
}];

As stated in #2 this would be done procedurally but for user defined reader macros this could be a huge usability win.

This could potentially also do away with the idea of dispatch macros.

const hashEntries = [{
  key: '#',
  action: readPrivateVars
}, {
  key: '#`',
  action: readSyntaxTemplate
}]

@disnet any thoughts?

[disnet]

This seems like a good idea to me.

I forget the motivation for Sweet using tries for keywords. Couldn't reading a keyword just be part of the readIdentifier action? It would just check the consumed string against a list of keywords and return a KeywordToken when it matches.

In any case, the need for multi-char keys seems obvious.

[gabejohnson]

I started using tries for punctuators to avoid writing the case analysis code. It was easy to reuse for keywords and I imagined there would be a performance benefit in the prop lookups vs switch + if statements though I've never tested it.

[disnet]

Ah, that makes sense.