Give NQPMu a default at_pos and at_key for type object invocants, like we have for Any in Rakudo.

Author: jnthn

src/core/NQPMu.pm

@@ -86,6 +86,16 @@ my class NQPMu {
         0
     }
 
+    proto method at_pos($pos) is parrot_vtable('get_pmc_keyed_int') { * }
+    multi method at_pos(NQPMu:U $self: $pos) {
+        NQPMu
+    }
+    
+    proto method at_key($key) is parrot_vtable('get_pmc_keyed_str') { * }
+    multi method at_key(NQPMu:U $self: $key) {
+        NQPMu
+    }
+    
     method defined() is parrot_vtable('defined') {
         nqp::isconcrete(self)
     }