Improve `URL` implementation

[QuiiBz]

The current URL implementation is fairly simple and isn't very compliant with the spec:

cd crates/wpt-runner && cargo run -- ../../tools/wpt/url/
...
449 tests, 38 passed, 410 failed
 -> 8% conformance

Most of the failing tests are located in urlencoded-parser.any.js (102 tests, 90 fails) and url-setters-stripping.any.js (260 tests, 260 fails)

• #835