Important mapper / matcher / anchoring refactorings

1. Decoupled the -mapper and -matcher libraries.
   (See https://github.com/hypothesis/dom-text-matcher/commit/41b3bff347a357eccb7bdb9eb5da224324e68b13 for more explanation)

2. Factor the -matcher initialization out of the strategies,
   since it's indepentend now.

3. Made two-phase anchoring mandatory for all document
   access strategies. (Codepath unification is underway.)
   This is experimantal and WIP. Range selector is disabled for now.