Make Associative <<op>> Associative work a bit better

- no longer throws if left side is an immutable type like Map
- directly call STORE rather than going through the assign magic
- call STORE with :INITIALIZE (so that it can work on immutable types)

This does *not* fix the issue with objects hashes and QuantHashes of the
keys are not strings.

src/core/metaops.pm6

@@ -540,22 +540,30 @@ multi sub HYPER(&op, Associative:D \left, Associative:D \right, :$dwim-left, :$d
         %keyset{$_} = 1 for right.keys;
     }
     my @keys = %keyset.keys;
-    my \result := left.WHAT.new;
-    result = quietly @keys Z=> HYPER(&op, left{@keys}, right{@keys}, :$dwim-left, :$dwim-right);
+    my \result := left.WHAT.new.STORE(
+      (quietly @keys Z=>
+        HYPER(&op,left{@keys},right{@keys},:$dwim-left,:$dwim-right)
+      ),
+      :INITIALIZE
+    );
     nqp::iscont(left) ?? result.item !! result;
 }
 
 multi sub HYPER(&op, Associative:D \left, \right, :$dwim-left, :$dwim-right) {
     my @keys = left.keys;
-    my \result := left.WHAT.new;
-    result = @keys Z=> HYPER(&op, left{@keys}, right, :$dwim-left, :$dwim-right);
+    my \result := left.WHAT.new.STORE(
+      (@keys Z=> HYPER(&op,left{@keys},right,:$dwim-left,:$dwim-right)),
+      :INITIALIZE
+    );
     nqp::iscont(left) ?? result.item !! result;
 }
 
 multi sub HYPER(&op, \left, Associative:D \right, :$dwim-left, :$dwim-right) {
     my @keys = right.keys;
-    my \result := right.WHAT.new;
-    result = @keys Z=> HYPER(&op, left, right{@keys}, :$dwim-left, :$dwim-right);
+    my \result := right.WHAT.new.STORE(
+      (@keys Z=> HYPER(&op,left,right{@keys},:$dwim-left,:$dwim-right)),
+      :INITIALIZE
+    );
     nqp::iscont(right) ?? result.item !! result;
 }