Fatal: Fix two mistakes with core subs.

Mistake 1: store the cached CORE:: sub under the right "name" in the
table (else it won't be used).

Mistake 2: Return the "original" sub instead of the wrapped sub when
using the cache.  This fixes a problem with cleaning up the cached sub
when they are unimported (for whatever reason).

Signed-off-by: Niels Thykier <niels@thykier.net>