Parse table-level `UNIQUE` constraints

[iafisher]

Example would be:

CREATE TABLE people(
  first_name TEXT,
  last_name TEXT,
  UNIQUE (first_name, last_name)
);

This enforces that no rows may have both the same first_name and the same last_name.

UNIQUE constraint can also take a conflict clause, like UNIQUE (first_name, last_name) ON CONFLICT ROLLBACK.

Full documentation: https://sqlite.org/lang_createtable.html

[luxint]

i've implemented this now at my fork, basically the same as the primary key table constraint

[iafisher]

Cool, if you have a PR I'd be happy to review it.