Expose match functionality of a rewrite-rule

[gramalingam]

It would be useful to expose the part of a rewrite-rule application that matches a pattern against a subgraph (rooted at a node) as standalone functionality. To enable this, first:

• Extract a base-class PatternImpl for class RewriteRule that encapsulates just the _target_pattern, _matcher, and _condition_function.
• Extract a base-class PatternBase for class RewriteRuleClassBase that encapsulates the pattern and check methods.

[justinchuby]

@gramalingam Do we want to expose the PatternImpl class? Or would there be a better name? Exposing a class with Impl seems a little strange.