Add RENAME CONSTRAINT

[shemgp]

I'm using an ERD software with apgdiff to generate migrations automatically for Laravel, and that software renamed all my constraints. Dropping and creating the constraints led to the diff not being importable in PostgreSQL, as is, due to dependencies in the constraints. Having apgdiff support renaming of constraints will remove the manual editing in my work flow. Thank you for considering this pull request.

[chadfurman]

I need to find time to pull this down and try it out. The more people the merrier