Parse_set / Parse_choice shrinking

[ampli]

(See PR #1419.)

1. Save 8 bytes in each.
2. On the same occasion add Parse_set::num_pc in unused space in Parse_set to save repeated counting in list_random_links().
3. Fix a bit rot in DEBUG_X_TABLE.

In principle, it is possible to calculate and cache num_pc in do_count() and then eliminate Parse_coice::next and simplify link extraction (no need to scan a list). There is a missing infrastructure for that in do_coutn() but I may add it (to speed up parsing with nulls). In any case, the overhead of this num_pc caching is unclear.