MySQL 8.0: Fix rare non-deterministic bug with index prefix key-parts
MySQL 8.0: Fix rare non-deterministic bug with index prefix key-parts

Bug: if a multi-column index uses prefix key parts (e.g. indexing only the
first N characters in a varchar column), Tengo's interpolation logic could
apply the prefix lengths to the wrong part of the index in MySQL 8.0. This
happened very rarely (only observed in a single test run, out of 100+ runs).

The root cause is that MySQL 8.0's new data dictionary does not guarantee
result order when querying information_schema.statistics. When a multi-col
index is returned, previous versions of MySQL would return the rows in
seq_in_index order per index. MySQL 8.0 almost always follows this order as
well, but apparently does not guarantee this.

This package already had logic to handle the out-of-order case, but it had a
subtle bug that was never triggered due to this code not previously
encountering the out-of-order condition from a live MySQL-server.

skeema/tengo,skeema/skeema