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.

src/core/Any.pm

@@ -14,7 +14,9 @@ my class Any { # declared in BOOTSTRAP
     ########
 
     # primitives
-    method infinite() { Nil }
+    method infinite()   { Nil }
+    method exists($key) { False }
+    method delete($key) { Nil }
     method list() {
         nqp::p6list(
           self.DEFINITE ?? nqp::list(self) !! nqp::list(), List, Mu