Add postcircumfix Map{} with list of keys.

Author: fluca1978

doc/Type/Map.pod6

@@ -26,7 +26,8 @@ To retrieve a value from the Map by key, use the C<{ }> postcircumfix
 operator:
 
     my $map = Map.new('a', 1, 'b', 2);
-    say $map{'a'}; # OUTPUT: «1␤»
+    say $map{'a'};      # OUTPUT: «1␤»
+    say $m{ 'a', 'b' }; # OUTPUT: «(1 2)␤»
 
 To check whether a given key is stored in a Map, modify the access
 with the C<:exists> adverb: