Add QuantHash.Setty|Baggy|Mixy coercer methods

These are basically frontends for coercing to the given object, but
with maintaining the mutability of the object.  So:

  say (1,2,3).Set.Baggy;       # Bag(1, 2, 3)
  say (1,2,3).SetHash.Baggy;   # BagHash(1, 2, 3)

src/core/Bag.pm6

@@ -43,6 +43,9 @@ my class Bag does Baggy {
         X::Immutable.new(method => 'DELETE-KEY', typename => self.^name).throw;
     }
 
+    method Setty(Bag:D:) { self.Set }
+    method Mixy(Bag:D:)  { self.Mix }
+
 #--- selection methods
     multi method grabpairs(Bag:D: $count?) {
         X::Immutable.new( method => 'grabpairs', typename => self.^name ).throw;

src/core/BagHash.pm6

@@ -66,6 +66,9 @@ my class BagHash does Baggy {
         )
     }
 
+    method Setty(BagHash:D:) { self.SetHash }
+    method Mixy(BagHash:D:)  { self.MixHash }
+
 #--- introspection methods
     method total() { Rakudo::QuantHash.BAG-TOTAL($!elems) }
 

src/core/Mix.pm6

@@ -23,6 +23,9 @@ my class Mix does Mixy {
         X::Immutable.new(method => 'DELETE-KEY', typename => self.^name).throw;
     }
 
+    method Setty(Mix:D:) { self.Set }
+    method Baggy(Mix:D:) { self.Bag }
+
 #--- introspection methods
     multi method WHICH(Mix:D:)    {
         nqp::if(

src/core/MixHash.pm6

@@ -65,6 +65,9 @@ my class MixHash does Mixy {
         )
     }
 
+    method Setty(MixHash:D:) { self.SetHash }
+    method Baggy(MixHash:D:) { self.BagHash }
+
 #--- object creation methods
     multi method new(MixHash:_:) { nqp::create(self) }
 

src/core/Set.pm6

@@ -106,6 +106,9 @@ my class Set does Setty {
     multi method DELETE-KEY(Set:D: \k) {
         X::Immutable.new(method => 'DELETE-KEY', typename => self.^name).throw;
     }
+
+    method Baggy(Set:D:) { self.Bag }
+    method Mixy(Set:D:)  { self.Mix }
 }
 
 # vim: ft=perl6 expandtab sw=4

src/core/SetHash.pm6

@@ -226,6 +226,9 @@ my class SetHash does Setty {
           )
         )
     }
+
+    method Baggy(SetHash:D:) { self.BagHash }
+    method Mixy(SetHash:D:)  { self.MixHash }
 }
 
 # vim: ft=perl6 expandtab sw=4