Add `Readtable.from` and `Readtable.of`

[gabejohnson]

Readtable.from :: Array<ReadtableEntry> -> Readtable and Readtable.of :: ReadtableEntry -> Readtable

Readtable.from([{
  key: '.',
  action(...) { ... }
}, {
   key: '/',
   action(...) { ... }
}]);

Readtable.of({ key: '?', action(...) { ... } });

If the signature of of was any -> Readtable instead, we could be https://github.com/fantasyland/fantasy-land#applicative compliant assuming the other required specifications were implemented.

Not sure of the utility of map and ap would be though.