WTS clean ups.

Move constants out of constructor so that we don't have to reassign them
every time we make a WikitextSerializer (which happens every time we
call WikitextSerializer#serializeHTML, which happens often when we call
WikitextSerializer#getAttribute{Key,Value}, etc).

Move set and map constants out of `_serializeAttributes` for similar
reasons.

Remove some duplicate code.

Remove the `options = options || {}` from the WikitextSerializer constructor
since the code as-is would crash if you didn't actually pass an options
object anyway (you need an `options.env`).

Change-Id: Id86d2e4b5833b9e900659f3812f441cf6d863c5c
Merge "WTS clean ups."