Final refactoring of psqlpac.rs

[justinlatimer]

• Create a Delta model which is generated by diffing a Project against a database. Contains a list of ChangeInstructions, can be saved to sql or a report.
• Move the logic to package a Project into the Project.
• Rename whats left in psqlpack.rs into an operation module, bringing that all back into one file.
• Move the fmtting of the AST elements into the ast module.

(the diff on this one is hard to read sorry, because moving generate_changeset from psqlpack to Delta made git think it's a file rename)

[justinlatimer]

This should finally close off #2

[paupino]

closes #2