C IsCOW_hek: detect 5.18 style IsCOW_hek, store shared_heks as hek

since 5.18 shared strings are mostly stored as COW heks.

store also in older versions shared heks (e.g. method names)
as hek not pv, so that the hek_hash and he_len access actually
is correct.
C 1.42_65 bump for IsCOW_hek
t/testc.sh: fix testcase 196