Reader modes

Adds support for reader modes

const noYs = getCurrentReadtable().extend({
  key: 'y',
  mode: 'terminating', // 'y' will now terminate Identifier parsing effectively acting as a delimiter
  action(stream) {
    throw Error('y is an invalid character');
  }
});

const useZs = getCurrentReadtable().extend({
  key: 'z',
  mode: 'non-terminating', // 'z' can still be used in Identifiers
  action: // ...
});

const colonese = getCurrentReadtable().extend({
  key: ':',
  mode: 'dispatch', // '#:' will dispatch to action and acting as 'non-terminating'
  action: // ...
});

// delegate mode in progress
const twoWaysToAdd = getCurrentReadtable().extend({
  key: 'Σ',
  mode: '+' // now Σ will return a '+' token,
  delegateTable: getCurrentReadtable()
});

I'm currently working on the delegate mode. I'm not sure how useful it's going to be and it does complicate things a bit.

I'm only including it b/c Racket has it. If I can't get it in a day or two I might drop it.