Implement Indexes

[paupino]

Currently indexes are implicitly ignored. Supporting indexes is important for psqlpack being used upon production databases.

[paupino]

• [x] Toposort
• [x] Validation
• [x] Delta
• [x] Integration Tests
• [x] Package write
• [x] Package from connection
• [x] Package from file

[paupino]

Indexes change slightly from 9.6 so it'd be useful to have #89 defined.