liburl: minor api changes

See commits for info, a number of these are 'breaking', although liburl is marked experimental so I'm not sure that matters so much.

First two commits will be impacted if #15138 is adopted, but it's a simple rename.
I think that the best convention for `&str => T` is via the `FromStr` trait, but sadly it throws away error information. I'm not sure we otherwise have conventions for string conversions, but perhaps these functions could be called `parse` because the string part is already implied?