Autogeneration of PrimaryKeyConstraint, fix undefined var typo

This pull request implements autogeneration for primary keys. One can change the name of the PK or the columns, and have it detected. Tests are in the class `test_autogenerate.AutogeneratePrimaryKeyTest`; the comments in `m5` explain the DDL changes expected, and this is tested in that class' `test_diffs`.

By the way, there is a typo in `autogenerate.compare._compare_uniques`: conn_cosntraint instead of conn_constraint. It's renamed in this.