Add Any.exists($key) and Any.delete($key)

This feels like a hack, but it seems the best way to handle cases such as

  my %h; say %h<a><b>:exists; say %h<a><b>:!k   # False, "b"

without having to add a large number of candidates for Any:U: in combination
with all possible adverbial pairs and combination of adverbial pairs.