Cannot parse f_rest_arg LITERAL_COMMA f_arg opt_f_block_arg

This is used in Method specs. We can go the same route as with block args and `for`, take a single splat argument in this case and do a multi-assignment. If we do that, however, I think we should refactor and do all argument passing like that

```ruby
module MethodSpecs

  class Methods
    def one_splat_one_req(*a,b); end
    def one_splat_two_req(*a,b,c); end
    def one_splat_one_req_with_block(*a,b,&blk); end

    def one_opt_with_stabby(a=->(b){true}); end
    
    define_method(:one_optional_defined_method) {|x = 1|}
  end
end
```