In rustdoc, the crate's own types don't need to be linked, but for some
web view that uses plain Markdown, the links looked weird. Fix this by
linkin the crates' own types too.
* Introduce and document the `url` feature
* Implement `From<&GeoUri>` and `From<GeoUri>` for `Url`
* Implement `TryFrom<&Url>` and `TryFrom<Url>` for `GeoUri`
* Add and extend tests
The uncertainty distance is a positive floating number.
* Validate this while parsing/building/using the setter
* Add the error variant `Error::OutOfRangeUncertainty` and change
`Error::InvalidDistance` to `Error::InvalidUncertainty`
* Update the tests