1.51_02: word-aligned PV overallocation >5.18, PMOP code_list

1. fixes t/CORE/comp/colon.t regression since 5.18, issue #372
COW Perl_sv_setsv_cow assert(SvCUR(sstr)+1 < SvLEN(sstr))
Fixed by overallocating each static string to at least word alignment.

Note: perl 5.18-5.21.3 itself has a broken overallocation scheme for short strings
(that's why perl5.20.1 disabled COW as performance regression),
because it allocates 10 byte instead of 8 for short strings len < 6.
We use 8 byte instead.

2. add prelim untested code for PMOP code_list, saveoptree for this OP*