interface_to_sv will soon start calling back into Perl, so any xsub invoking it
(directly or indirectly) needs to save the stack pointer via PUTBACK/SPAGAIN.

Currently, arg_to_sv and get_field are the only affected functions that are
called from xsubs.  We provide macros SS_arg_to_sv and SS_get_field that
automatically handle the stack pointer correctly.