Fatal: Smarter handling of CORE subs in nested autodie

Previously, when nesting autodie with itself, autodie would wrap its
own wrapper.  With this patch, autodie properly detects it is dealing
with a "wrapped CORE sub" and simply generates a normal CORE wrapper.

Besides removing one level of indirection, it also removes the
overhead of checking for hints for the CORE sub (which was somewhat
counterintuitive, since autodie normally ignores hints for CORE subs).

Signed-off-by: Niels Thykier <niels@thykier.net>
Fatal: Remove unused local of $" and $! under autodie

The use of local on this variables have no apparent visible effect at
all (except hiding $! for callers).  We retain it for Fatal, where at
least the modified value of $" is still used by the wrapper.

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