Make Junction.Str return...a Str!

Previously, it would form a Junction with everything Str'd. Thing is,
things that (fairly) expect to get back a Str when they call .Str will
then explode when they try to unbox it. This brings us in line with
Niecza's semantics, so at the very least it's convergence...

src/core/Junction.pm

@@ -20,7 +20,7 @@ my class Junction is Mu {
     }
 
     multi method Str(Junction:D:) {
-        self.new($!storage.map({$_.Str}), :type($!type))
+        self.perl
     }
 
     multi method ACCEPTS(Junction:D: Mu \topic) {