Adds support for working with URL Paths

It is sometimes useful to parse just the path portion of a URL (path,
query string and fragment) rather than the entire URL.
auto merge of #12231 : wycats/rust/url_path_parse, r=alexcrichton

It is sometimes useful to parse just the path portion of a URL (path,
query string and fragment) rather than the entire URL.

In theory I could have made Url embed a Path, but that would be a
breaking change and I assume that Servo uses this API. I would be
happy to update the PR to embed Path in Url if that's what people
wanted.