Fix #729. A different approach:

For strings, object_id can be simply their string representation. For
booleans, numbers, and nil, follow the MRI scheme:

* false: 0
* true: 2
* nil: 4
* numbers: (n * 2) + 1
* heap objects: 6, 8, 10, 12, 14, 16, etc…

* Test coverage for object_id for true, false, nil, numbers, and strings
* Test coverage for Opal.uid()
* Make sure Opal.uid() starts incrementing from 4 (initialize the
unique_id var from the nil_id var, then piggy-back on the fact that
nil.object_id == 4 is already covered by the NilClass#object_id spec)
* Fix bug with unique_id being accessed directly by
setup_module_or_class_object in runtime.js. All access to the unique_id
var shall be via Opal.uid()

Experiment 2: define object_id for strings, numbers, and booleans