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
Exclude stubs and non-functions from Module#instance_methods
Update compiler specs
Use #hash to generate internal Hash map keys
Improve Array#hash implementation

Still not foolproof tho.
Explicitly set nil’s object_id

Now’s different from the id of NilClass.
Reimplement Hash#merge in terms of clone+merge
Use more advanced Kernel & Numeric #hash functions
Cleanup rake mspec_node task
Use my rubyspec fork until it’s merged
Allow passing an external pattern to mspec_node
Compare using #eql? during #hash clashes in #key?
Handle recursive values and keys in Hash#inspect
Mark the non compliant parts in rubyspecs
Use BasicObject#hash For Hash#hash
Ensure the $$id is generated via __id__