Cleanup - break positional requirement of keywords

[happy-barney]

Proper behaviour of op.c depended on correct keyword order in regen/keywords.pl.

Affected logic was fragile and magic (% 3).

Change breaks such requirement, allowing alphabetical sorting (always improving maintenance).

• Primary test

• t/op/coreamp.t

• This set of changes does not require a perldelta entry.

• Follow-ups

• improve documentation why some keywords are + and another -
• use PPI to project changes to lib/B/Deparse.pm