Ruby

Consolidate empty keyword handling

Remove rb_add_empty_keyword, and instead of calling that every
place you need to add empty keyword hashes, run that code in
a single static function in vm_eval.c.

Add 4 defines to include/ruby/ruby.h, these are to be used as
int kw_splat values when calling the various rb_*_kw functions:

RB_NO_KEYWORDS :: Do not pass keywords
RB_PASS_KEYWORDS :: Pass final argument (which should be hash) as keywords
RB_PASS_EMPTY_KEYWORDS :: Add an empty hash to arguments and pass as keywords
RB_PASS_CALLED_KEYWORDS :: Passes same keyword type as current method was
called with (for method delegation)

rb_empty_keyword_given_p needs to stay. It is required if argument
delegation is done but delayed to a later point, which Enumerator
does.

Use RB_PASS_CALLED_KEYWORDS in rb_call_super to correctly
delegate keyword arguments to super method.