Support Expressions as index fields

[Veesh Goldman]

          @rabbiveesh As you design that, an interesting unit test for postgres to really test the boundaries would be

create table test (name varchar(50) not null);
create index ix_test2 on test (substr(name, 2, 3) desc, substr(name, 5, 3) asc);

:-)

Originally posted by @nrdvana in #68 (comment)

There are a few more comments in that PR's discussion. (will past them back here).

Another thought - when parsing the fields, sqlt helpfully splits strings on commas. So we're actually going to have to only support that via a hashref arg (as mentioned there)

ref #68 (comment)
and #68 (comment)