Various runtime cleanups

Remove usage of $opal in favor of Opal.

Rename Opal.$return to Opal.ret.

Rename Opal.$yield1 to Opal.yield1.

Rename Opal.$yieldX to Opal.yieldX.
Add stubs only when there are some
Support anonymous class inspection in BasicObject#method_missing error
Fix Module#instance_methods for modules
Cleanup Opal.rescue