Fix crash in Proxy.perl

Phixes #1466

The crash occurs due to `self` of a Proxy-ed object being conted,
but access to it fetches the value. So we end up entering .perl
via Mu:D candidate (the Proxy object itself) and then calling
.perlseen method on it (thus on the fetched value). So if that value
is a :U, then we explode, because there ain't no :U .perlseen candidate.
If the value is a :D, we'd generate incorrect .perl for many types.

Fix by checking if `self` is conted to discern when we're dealing
with a Proxy and then calling .perl on the fetched value. Not 100% sure
it should behave like that, rather than giving some sort of
"Proxy.new: …" value.

src/core/Mu.pm

@@ -642,15 +642,17 @@ Perhaps it can be found at https://docs.perl6.org/type/$name"
         nqp::if(
           nqp::eqaddr(self,IterationEnd),
           "IterationEnd",
-          self.perlseen(self.^name, {
-              my @attrs;
-              for self.^attributes().flat.grep: { .has_accessor } -> $attr {
-                  my $name := substr($attr.Str,2);
-                  @attrs.push: $name ~ ' => ' ~ $attr.get_value(self).perl
-              }
-              self.^name ~ '.new' ~ ('(' ~ @attrs.join(', ') ~ ')' if @attrs)
-          })
-        )
+          nqp::if(
+            nqp::iscont(self), # a Proxy object would have a conted `self`
+            nqp::decont(self).perl,
+            self.perlseen: self.^name, {
+                my @attrs;
+                for self.^attributes().flat.grep: { .has_accessor } -> $attr {
+                    my $name := substr($attr.Str,2);
+                    @attrs.push: $name ~ ' => ' ~ $attr.get_value(self).perl
+                }
+                self.^name ~ '.new' ~ ('(' ~ @attrs.join(', ') ~ ')' if @attrs)
+            }))
     }
 
     proto method DUMP(|) {*}