Copy hints from tied hh to inner compile scopes

Copy hints from tied hh to inner compile scopes

Entries from a tied %^H were not being copied to inner compile-time
scopes, resulting in %^H appearing empty in BEGIN blocks, even
though the underlying he chain *was* being propagated properly (so
(caller)[10] at run time still worked.

I was surprised that, in writing tests for this, I produced another
crash.  I thought I had fixed them with 95cf23680 and 7ef9d42ce.  It
turns out that pp_helem doesn’t support hashes with null values.
(That’s a separate bug that needs fixing, since the XS API allows for
them.)  For now, cloning the hh properly stops pp_helem from getting a
null value.