Add Url::make_relative() as the inverse of Url::join()

[sdroege]

This creates a relative URL from a base URL and another absolute URL.

Fixes #666

[sdroege]

The clippy failures are all unrelated to this PR. I can submit another PR for them if you agree that all those should be fixed, or let me know which of the lints you'd like to disable (personally I'm not a fan of clippy::manual-range-contains...)

[djc]

Yup, sorry about the clippy failures. Another PR would be nice, though a separate commit in this PR is also fine. I think we should fix all of them except the ones about the unit errors, because those would break backwards compatibility.

[sdroege]

clippy fixes are in #669

[sdroege]

Rebased on top of #669

[valenting]

Thanks for this PR. It looks really nice.
Would you mind adding a couple of tests for the negative? ie? returning None?

[sdroege]

Sure, will add a few tests for that later or tomorrow! Thanks for the review

[sdroege]

Added tests for the 4 possible error cases.

[bors-servo]

☔ The latest upstream changes (presumably #703) made this pull request unmergeable. Please resolve the merge conflicts.

[sdroege]

And rebased for the merge conflicts. @valenting anything else missing here?