sql: support expression-based index columns

[knz]

Computed indexes are indexes on the result of an expression (as opposed to the direct value of a column). For example: CREATE INDEX a ON customer( LOWER(firstname) || ' ' || LOWER(lastname))

Suggested by @petermattis

[knz]

Would help with #7186 too.

[awoods187]

This is one of the top unimplemented errors encountered by users