Previously, proc calls such as: ```ruby proc{|| }.(**empty_hash) proc{|b: 1| }.(**r2k_array_with_empty_hash) ``` both allocated hashes unnecessarily, due to two separate code paths. The first call goes through CALLER_SETUP_ARG/vm_caller_setup_keyword_hash, and is simple to fix by not duping an empty keyword hash that will be dropped. The second case is more involved, in setup_parameters_complex, but is fixed the exact same way as when the ruby2_keywords hash is not empty, by flattening the rest array to the VM stack, ignoring the last element (the empty keyword splat). Add a flatten_rest_array static function to handle this case. Update test_allocation.rb to automatically convert the method call allocation tests to proc allocation tests, at least for the calls that can be converted. With the code changes, all proc call allocation tests pass, showing that proc calls and method calls now allocate the same number of objects. I've audited the allocation tests, and I believe that all of the low hanging fruit has been collected. All remaining allocations are either caller side: * Positional splat + post argument * Multiple positional splats * Literal keywords + keyword splat * Multiple keyword splats Or callee side: * Positional splat parameter * Keyword splat parameter * Keyword to positional argument conversion for methods that don't accept keywords * ruby2_keywords method called with keywords Reapplies abc04e898b627ab37fa9dd5e330f239768778d8b, which was reverted at d56470a27c5a8a2e7aee7a76cea445c2d29c0c59, with the addition of a bug fix and test. Fixes [Bug #20679]