Mark Scalar as never being repossessed

This restores a somewhat dubious, but relied upon, pre-rescalar
behavior. A previous attempt at fixing #2056 led to #2126. Together
with the revert in NQP commit 9fe83eb3e9, the modules in both tickets
now all work.

src/Perl6/Metamodel/BOOTSTRAP.nqp

@@ -1581,6 +1581,9 @@ BEGIN {
     }));
     Scalar.HOW.compose_repr(Scalar);
 
+    # To preserve historical behavior, we never repossess a Scalar container.
+    nqp::neverrepossess(Scalar);
+
     # Scalar needs to be registered as a container type. Also provide the
     # slow-path implementation of various container operations.
     nqp::setcontspec(Scalar, 'rakudo_scalar', nqp::hash(