Add support for function indexes

[wilj]

Request: Add support for MySQL function/expression indexes such as:

CREATE INDEX idx1 ON t1 ((col1 + col2));
CREATE INDEX idx2 ON t1 (lower(trim(col3)));

[timsehn]

Thanks. We will investigate and get back to you.

As I said in the other ticket, materializing this as a column with a trigger is the workaround.

[zachmu]

This is a relatively large feature and will take quite a bit of work to implement. I'm adding it to the roadmap.