Mark set()/bag()/mix() as pure

So that set/bag/mix with literal values can be created once at compile
time, instead of at runtime.

src/core/set_operators.pm6

@@ -1,12 +1,12 @@
-proto sub set(|) {*}
+proto sub set(|) is pure {*}
 multi sub set() { BEGIN nqp::create(Set) }
 multi sub set(*@a --> Set:D) { Set.new(@a) }
 
-proto sub bag(|) {*}
+proto sub bag(|) is pure {*}
 multi sub bag() { BEGIN nqp::create(Bag) }
 multi sub bag(*@a --> Bag:D) { Bag.new(@a) }
 
-proto sub mix(|) {*}
+proto sub mix(|) is pure {*}
 multi sub mix() { BEGIN nqp::create(Mix) }
 multi sub mix(*@a --> Mix:D) { Mix.new(@a) }