MySQL 8 index expressions: fix edge case panic
MySQL 8 index expressions: fix edge case panic

Recent commit 515748af added support for MySQL 8.0 index expressions. In
tables containing multiple secondary indexes, the method fixIndexOrder would
panic if at least one index contained an index expression at the front of the
index.

The root cause was a regular expression that strictly expected a column name,
failing to match a paren-wrapped expression.

skeema/skeema,skeema/tengo