Handle forwarding parameters in parser

[Morriar]

Summary

Expand ... (ForwardingParameterNode) into *args, **kwargs, &block instead of crashing with an unexpected parameter error.

Based on #562 by @kddnewton — simplified the approach to avoid wrapping every case in arrays: just map → flat_map and a single new when branch.

Includes test for both def m1(...) and def m2(a, ...).

Closes #562

[vinistock]

I think the changes are fine, but I do want to confirm one aspect of this. There one minor difference before ... and the expansion, which is the fact that you cannot refer to specific parameter parts when using .... For example:

def foo(...)
  # there's no way to use forwarding parameters that isn't including all
  # of them. I can't print only the keyword ones or refer exclusively to the block
end

def bar(*args, **kwargs, &block)
  # Here, while equivalent at the call site (accepts any argument), I can refer
  # to the individual parameter parts
end

Is this something we need to be concerned about? Would it be better to have RBI print the forwarding parameter instead?