Allow calling .DEPARSE with a class

This allows for subclassing RakuAST::Deparse for customization.

    use experimental :rakuast;
    class A is RakuAST::Deparse { }
    say Q/say "foo"/.AST.DEPARSE(A);  # say("foo")

src/Raku/ast/base.rakumod

@@ -289,10 +289,13 @@ class RakuAST::Node {
 
     # Hook into the Raku RakuAST::Deparse class (by default) or any other
     # class that has been put into the hllsym hash for 'Raku'
-    method DEPARSE() {
+    method DEPARSE($class?) {
         # TODO: figure out how to take a single optional positional that
         # expects a class, and how to check for that here.
-        nqp::gethllsym('Raku','DEPARSE').deparse(self)
+        ($class =:= NQPMu
+          ?? nqp::gethllsym('Raku','DEPARSE')
+          !! $class
+        ).deparse(self)
     }
 
     method IMPL-SORTED-KEYS(Mu $hash) {