All indentation was normalized by rubocop auto-correct at 80e66cc4.
But heredocs was still kept absolute position. This commit aligns
heredocs indentation for consistency.

As part of the larger refactoring to remove type casting from Arel, we
need to do the casting of values eagerly. The predicate builder is the
closest place that knows about the Active Record class, and can
therefore have the type information.

/cc @mrgilman

[Sean Griffin & Melanie Gilman]

This will allow us to pass the predicate builder into the constructor of
these handlers. The procs had to be changed to objects, because the
`PredicateBuilder` needs to be marshalable. If we ever decide to make
`register_handler` part of the public API, we should come up with a
better solution which allows procs.

/cc @mrgilman

[Sean Griffin & Melanie Gilman]