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 abc04e89, which was reverted at
d56470a2, with the addition of a bug fix and
test.

Fixes [Bug #20679]