Add support for creating unique indexes

[kvch]

This PR adds a new attribute to create_index operation.
From now on it is possible to create a unique index on one or more
columns in a table.

[andrew-farries]

We have a few ways of ways to make unique columns now:

• setting unique: true on a column when it's created: [example]
• using an alter column operation to set a column as unique: [example]

And now creating unique indexes as of this PR.

I think this is fine. One thing to be aware of is that we don't currently duplicate indexes on a column when duplicating it [see https://github.com//issues/227], so if we have a unique index on a column and and then add, for example, a NOT NULL constraint to that column, the unique index will be lost.