Implement .grabpairs where they're *not* supposed to work

src/core/Bag.pm

@@ -33,6 +33,9 @@ my class Bag does Baggy {
     method grab($count = 1 --> Int) is hidden_from_backtrace {
         X::Immutable.new( method => 'grab', typename => self.^name ).throw;
     }
+    method grabpairs($count = 1 --> Int) is hidden_from_backtrace {
+        X::Immutable.new( method => 'grabpairs', typename => self.^name ).throw;
+    }
 
     method Bag     { self }
     method BagHash { BagHash.new-fp(nqp::getattr(self, Bag, '%!elems').values) }

src/core/Mix.pm

@@ -23,6 +23,9 @@ my class Mix does Mixy {
     method grab($count = 1 --> Real) is hidden_from_backtrace {
         X::Immutable.new( method => 'grab', typename => self.^name ).throw;
     }
+    method grabpairs($count = 1 --> Real) is hidden_from_backtrace {
+        X::Immutable.new( method => 'grabpairs', typename => self.^name ).throw;
+    }
 
     method Mix { self }
     method MixHash { MixHash.new-fp(nqp::getattr(self, Mix, '%!elems').values) }

src/core/Set.pm

@@ -24,6 +24,9 @@ my class Set does Setty {
     method grab ($count = 1) {
         X::Immutable.new( method => 'grab', typename => self.^name ).throw;
     }
+    method grabpairs ($count = 1) {
+        X::Immutable.new( method => 'grabpairs', typename => self.^name ).throw;
+    }
 
     method Set { self }
     method SetHash { SetHash.new(self.keys) }