Unify the two pattern-matchers (#1495)

Update generic pattern matcher to support the unified API.

* Update generic_pattern_test cases to use unified API.
* Update the generic_pattern implementation.

Still todo:
* generic_pattern doesn't support attributes yet
* pattern hasn't been updated yet to support extra first-parameter for
pattern-builder and checker-function
* unify PatternMatchResult and MatchResult
* More redundancies to be eliminated between the two matchers. Eg.,
generic-pattern doesn't yet update opset-imports (unlike pattern)