Proposal: Add version diff

`go-version` currently supports comparing versions as a whole, e.g. `2.3.4` > `1.2.3`. Sometimes we need to determine if the difference between versions is a major change, a minor change, etc.

Since `Compare()` supports an arbitrary number of components, the diff should as well.

Only the first diff is likely to be relevant, as in the diff of `2.3.4` vs `1.2.3`, we care about the major version change, and the minor version changes are not meaningful.

I can work on this if this sounds good. We're going to be using a very pared-down version of this in the AWS Provider.