Added more tests for isIn so that in addition to being tested under
CheckedEquality it was also tested under Unchecked- and EnabledEquality.
The EnabledEquality one had some compiler errors because some things
weren't enabled. Took me a while to figure out what was not enabled, but
one thing was Tuple2, which is needed for Scala Maps, and java.util.Map.Entry,
which is needed for Java Maps. Along the way I noticed that should === syntax
was still requiring a Constraint instead of an EqualityConstraint. Fixing that
brought to the surface the need to make some conversions from Equivalence to
EqualityConstraint (for explicitly DSL or otherwise explicitly provided
Equivalences) implicit under the old policies, which brought to the surface
the need to move pairs of them up to LowPriority traits. All and all a fine
day of yak shaving, with the end result that the isIn syntax is tested
under all policies, and ready for indexIn and lastIndexIn.