Add 'table' property to Readtable entries

[gabejohnson]

It would be useful to generalize dispatch macros and not have to hard-code a single character as the dispatch character.

For instance:

const entry = {
  key: '#',
  table: Readtable.from([...]),
  action(...) { ... }
}

The semantics would be defined in the reader but I'm thinking:

1. If the entry has a table, look at the next char and see if the table has an entry for it and recurse.
2. If the entry has a table and an action, do 1 but only after running the action (for side-effects/setup).
3. If the entry has no table, just execute the action.

Maybe 2 is unnecessary or maybe the action just before the final table found should execute.

Motivation:

The current table-driven reader in sweet.js uses some simple tries for dealing w/ punctuators and keywords. This function could be built into the readtable implementation.

const tEntry = {
  key: 't',
  table: Readtable.from([{
    key: 'h',
    table: Readtable.from([{
      key: 'i',
      table: Readtable.from([{
        key: 's',
        action(stream) {
          stream.readString();
          return KeywordToken({ value: 'this' });
        }
      }])
    }, {
      key: 'r',
      table: Readtable.from([{
        key: 'o',
        table: Readtable.from([{
          key: 'w',
          action(stream) {
            stream.readString();
            return KeywordToken({ value: 'throw'});
          }
        }])
      }])
    }])
  }])
};

Now this would obviously be procedurally generated, but this illustrates the concept.

Note: Another way to achieve similar functionality would be to allow multi-char keys.

[gabejohnson]

Another possible use could be to have the reader set table as the current readtable. This could simplify certain reader implementations a great deal.

const stringTableEntries = [{
  action: function(stream) {
    return stream.readString() + this.read(stream);
  }
}, {
  key: "'",
  action: function() { return ''; }
}, {
  key: '\\',
  action: readStringEscape
}];

function readStringLiteral(stream) {
  return StringToken({ str: this.read(stream) });
}

const stringEntries = [{
  key: "'",
  table: stringTable,
  action: readStringLiteral
}, {
  key: '"',
  table: stringTable,
  action: readStringLiteral
}];

/cc @disnet