Optimize minimum requirements further

[dagolden]

This commit improves optimization for the case where add_minimum gets a version object that stringifies to "0".

It also optimizes the _version_object method to avoid expensive magic checks for "0" or version objects.

Some comments were added to clarify why things are optimized the way they are.

[dagolden]

@rjbs, I'd appreciate a third set of eyes before I merge this. (@leont already approves)

[rjbs]

:+1: