WTS Code cleanup: Consistently use serializer instead of prototype.

* A lot of code was referencing the serializer prototype rather
  than the serializer object constructed from the prototype.
  Discovered when a tracing call protected with (this.debugging)
  failed because 'this' was the prototype rather than the
  serializer object.

* Also allocate a wt-escaping handler object in the serializer
  constructor instead of assigning one in the serializer prototype.

* No change in parser tests results.

Change-Id: I181abc32ad0c73f3919f2476e8b852885556846f