Support forwarded arguments in calls, super, and nested blocks

[pvcresin]

Known issue: forwarding-arguments

Summary

• support argument forwarding with ... in direct calls, super(...), and nested blocks
• move the forwarding-arguments case from known issues to regular scenarios
• add scenario coverage for bar(...), super(...), and 1.times { bar(...) }

Design

• model ... as forwarded actual arguments instead of reconstructing it from local variables
• keep the forwarding state in a dedicated object referenced from LocalEnv
• reuse the same forwarding state for super(...) and nested block bodies

Note

The implementation is based on the existing argument-binding logic in MethodDefBox#pass_arguments.

ForwardingArguments#accept_actual_arguments mirrors that flow to capture the actual arguments received by def foo(...), and ForwardingArguments#to_actual_arguments converts that captured state back into ActualArguments for direct calls, super(...), and nested block calls.

[mame]

Thanks @pvcresin! Clean design and the test coverage for the three cases is convincing.

Does bar(extra, ...) work? In CallBaseNode#install0, when @forwarding_arguments is true, @positional_args looks discarded entirely. Probably out of scope for this PR. Could you drop it into scenario/known-issues/ so we don't lose track?

[pvcresin]

@mame Good catch, and you're right: bar(extra, ...) does not work on this branch because the leading positional args are dropped when @forwarding_arguments is true.

I've added a regression scenario under scenario/known-issues/ so we don't lose track of it:
scenario/known-issues/forwarding-arguments-extra-positional.rb

I also rebased the branch onto the latest master.

[mame]

Thanks!