Fix handling of generics in TypedArray.raku

When calling .raku on a generic typed array, it's possible for the
method to wind up calling .raku on an uninstantiated generic. Instead of
using the typed array's parameter directly, use the type of the array's
container descriptor (which should get instantiated eventually).

Fixes #3784

src/core.c/TypedArray.pm6

@@ -104,12 +104,12 @@
             )
         }
 
-        multi method raku(::?CLASS:D \SELF:) {
-            my $type := TValue.raku;
+        multi method raku(::?CLASS:D:) {
+            my $type := nqp::getattr(self,Array,'$!descriptor').of.^name;
             my $raku := self.map({
                 nqp::isconcrete($_) ?? .raku(:arglist) !! $type
             }).join(', ');
-            'Array[' ~ $type ~ '].new(' ~ $raku ~ ')';
+            'Array[' ~ $type ~ '].new(' ~ $raku ~ ')'
         }
     }
     method ^parameterize(Mu:U \arr, Mu \t, |c) {