Implement ADD INDEX and DROP INDEX (#196)

This PR implements `ADD INDEX` and `DROP INDEX` statements in the new
driver.

It was necessary to reorganize some bits of the information schema
builder, as indexes and constraints overlap in both the grammar (ALTER
TABLE uses a `tableConstraintDef` node for both indexes and
constraints), and in reality (UNIQUE index needs a constraint, UNIQUE
constraint needs an index, making them equivalent).

Author: adamziel