Making this implement Function was a mistake; it should use delegation
rather than inheritance. Pass in a Java 8 Function instead of using
Guava.
Avoid some unnecessary materialization and help downstream callers by
returning a Stream. Inline the hasMatch method to the one place it's
used, since the corresponding Stream operation is already clear enough.
Clarify the implementation in the process of changing the types.
However, do not attempt to change the actual algorithm, under the
assumption (dating to I12cad5e5) that this is already performance-
sensitive and shouldn't be changed without appropriate benchmarking.
Change-Id: Iafd5e9638cd85c5358947b0ec7172362ecde678d
3938c1726f