Ensure f(**kw, &block) calls kw.to_hash before block.to_proc

Previously, block.to_proc was called first, by vm_caller_setup_arg_block.
kw.to_hash was called later inside CALLER_SETUP_ARG or setup_parameters_complex.

This adds a splatkw instruction that is inserted before sends with
ARGS_BLOCKARG and KW_SPLAT and without KW_SPLAT_MUT. This is not needed in the
KW_SPLAT_MUT case, because then you know the value is a hash, and you don't
need to call to_hash on it.

The splatkw instruction checks whether the second to top block is a hash,
and if not, replaces it with the value of calling to_hash on it (using
rb_to_hash_type).  As it is always before a send with ARGS_BLOCKARG and
KW_SPLAT, second to top is the keyword splat, and top is the passed block.

test/ruby/test_call.rb

@@ -147,6 +147,23 @@ class TestCall < Test::Unit::TestCase
     assert_equal Hash, f(*[], **o).class
   end
 
+  def test_kwsplat_block_order
+    o = Object.new
+    ary = []
+    o.define_singleton_method(:to_a) {ary << :to_a; []}
+    o.define_singleton_method(:to_hash) {ary << :to_hash; {}}
+    o.define_singleton_method(:to_proc) {ary << :to_proc; lambda{}}
+
+    def self.t(...) end
+
+    t(**o, &o)
+    assert_equal([:to_hash, :to_proc], ary)
+
+    ary.clear
+    t(*o, **o, &o)
+    assert_equal([:to_a, :to_hash, :to_proc], ary)
+  end
+
   OVER_STACK_LEN = (ENV['RUBY_OVER_STACK_LEN'] || 150).to_i # Greater than VM_ARGC_STACK_MAX
   OVER_STACK_ARGV = OVER_STACK_LEN.times.to_a.freeze