Add enumerable_method? for MethodIdentifierPredicates

[fatkodima]

For context - https://github.com/rubocop-hq/rubocop-performance/pull/140/files#r439732580

This was already needed several times: one, two

[marcandre]

Good work. Another good example for #29 to avoid 61 loops all the time.

[Drenmi]

There's a small chance this might cause an issue due to a mismatch between Ruby version RuboCop is run with versus the version of the code inspected, but I think it's a small enough concern that this is still okay.

[marcandre]

That's a good point. Might be best to list them explicitly.