Optimize super as a bareword

[jgaskins]

Currently, it's hardcoded in the compiler to use $slice.call(arguments, 0). This PR just does the usual arguments->array copy via iteration while also making the JS code less terrible than I've been doing by declaring that target variable with the rest of the locals. :-)

Results from rake bench (from left to right: master, this PR, MRI):

test/cruby/benchmark/bm_vm2_zsuper.rb              13.864  1.780   0.547

As a bonus, I renamed the index variable for splat args and added a local variable for it. I hadn't intended to commit it but I apparently wasn't paying attention. :-) The only reason I changed it at all was because I originally used arg_index in both places, but Struct.new uses both splat args and bareword super so var arg_index in two places within the same scope caused JSHint to barf.

[elia]

I'd use "$zuper_index" and "$splat_index" to avoid potential conflicts with local vars

[jgaskins]

Ah, good idea.

[jgaskins]

Alrighty, preceding dollar signs added, ready to go.

[meh]

👍