Duplicate unique and check constraints correctly

[kvch]

Previously, unique and check constraints with multiple columns were not duplicated correctly.

We had two issues:

1. Pgroll tried to create the same duplicated index for each renamed column. From now on pgroll creates the index once with the new column names.
2. Pgroll tried to convert the unique index to a constraint multiple times. From now on, the index is only converted once.

Required by #464

[kvch]

It is highly likely we need to adjust the check constraint migrations as well. I will address those issues in a follow-up PR.

[kvch]

I pushed the multi-column duplicator change. I also tested it with the new check constraint PR and it fixes the problem with the example.

[kvch]

I have split out the statement builder from the duplicator and added unit tests to make sure we generate the correct SQL statements for constraints.

[andrew-farries]

This looks good. Thanks for adding the tests for the duplicators.

Would it be possible to combine the ColumnDuplicator and the ColumnGroupDuplicator so that a duplicator could be created with one or more columns?

[andrew-farries]

Great 🎉

Approved with one more suggesion. 👍

[kvch]

Updated the PR with the suggestion, and merging once the tests pass.