[bug] Unique index (Primary Key) is not working in the example.

[uds5501]

Extending the example given in the readme, I attempted the following -

Limbo v0.0.10
Enter ".help" for usage hints.
limbo> CREATE TABLE users (id INT PRIMARY KEY, username TEXT);
limbo> INSERT INTO users VALUES (1, 'alice');
limbo> INSERT INTO users VALUES (1, 'abra'); // should've failed.
limbo> select * from users;
1|alice
1|abra

This should've failed to insert (1, 'abra') due to unique constraint but it didn't.
Would be happy to pick the fix if it's indeed an error btw :)

[jussisaurio]

Duplicate of #472