Move #helper to API, simplify #let on endpoint

Calling #helper directly on Endpoint doesn't make much sense, since
you can simply define methods if you've subclassed Endpoint. It's
really only something that's used as an API DSL method.

Now that we're class_eval'ing the helper block directly on the
endpoint subclass, we can support defining lets in the helper block.

Signed-off-by: Stephen Celis <stephen.celis@gmail.com>
Signed-off-by: Evan Owen <kainosnoema@gmail.com>