Add Perl_amagic_deref_call() to implement the bulk of tryAMAGICunDEREF_var().

Add Perl_amagic_deref_call() to implement the bulk of tryAMAGICunDEREF_var().

This removes around 300 bytes of object code from each place it was previously
inlined. It also provides a better interface - quite a lot of the core
currently bodges things by creating a local variable C<SV **sp = &sv> to use
the macro.

Change the XS::APItest wrapper to amagic_deref_call().