This commit makes two performance tweaks to the dispatch_cache in
Arel::Visitors::Visitor.

First, it interns the method name into a symbol rather than a string.
send expects a symbol, so this avoids Ruby needing to intern that string
itself.

Second, it changes the dispatch cache to an identity hash. Classes are
already compared by identity, but Ruby is able to perform the lookup
more quickly if it is explicit on the hash.