REF: simplify should_extension_dispatch, remove dispatch_to_extension_op
dispatch_to_extension_op was needed back before we got rid of integer-addition for DTA/TDA/Timestamp/Timedelta.  Now it reduces to a one-liner, which this inlines.

This also consolidates all of the casting/checks up-front, which in turn avoids a runtime import and simplifies the check for should_extension_dispatch.