1.48: Support 5.20

Support the 5.20 changes so far (BIND vs INVLIST)
Simplify tautological (sv)->sv_flags != SVTYPEMASK to !SvIS_FREED(sv)

Improve t/basic.t a bit
Updated t/op_list.t for 5.18 1 and 5.19: padrange, kvaslice, kvhslice

Failing versions: 5.17.8, 5.19.2 sprout/postderef