[ 10564]
Avoid AV and HV in perlio.c by inventing PerlIO_list_t which is AV-ish
and using that instead (name lookups are sequential search for now).
p4raw-link: @10564 on //depot/perlio:
fcf2db383b9625d65c84a8308e9be05a073bed3b
p4raw-id: //depot/perl@10567