Put BUILD in Baggy/Setty role, so we can further simplify KeyBag/KeySet

src/core/Baggy.pm

@@ -2,6 +2,8 @@ my role Baggy does Associative {
     has %!elems; # should be UInt
 
 #- specific methods for users of role ------------------------------------------
+    method BUILD (:%!elems) {}
+
     method at_key($k) {
         Proxy.new(
           FETCH => {

src/core/KeyBag.pm

@@ -1,9 +1,5 @@
 my class KeyBag does Baggy {
 
-    submethod BUILD (:%elems)  {
-        nqp::bindattr(self, KeyBag, '%!elems', %elems);
-    }
-
     method Bag { Bag.new-fp(nqp::getattr(self, KeyBag, '%!elems').values) }
     method KeyBag { self }
 }

src/core/KeySet.pm

@@ -1,9 +1,5 @@
 my class KeySet does Setty {
 
-    submethod BUILD (:%elems)  {
-        nqp::bindattr(self, KeySet, '%!elems', %elems);
-    }
-
     method Set { Set.new(self.keys) }
     method KeySet { self }
 }

src/core/Setty.pm

@@ -2,6 +2,8 @@ my role Setty does Associative {
     has %!elems;
 
 #- specific methods for users of role ------------------------------------------
+    method BUILD (:%!elems) {}
+
     method at_key($k --> Bool) {
         Proxy.new(
           FETCH => {