simplified approach to ensuring Url type is returned from urlAssembly

[stackoverfloweth]

In an attempt to simplify common use cases of route.matches, we discovered some functions that were returning string when they should be returning Url.

In a previous approach, we put it on the developer to make sure the routes they built satisfy the Url type when assembled. However, we couldn't really test routes since routes might have dynamic params. Plus developers probably aren't super familiar with what it means for a route to be "assembled".

This PR takes a much simpler approach by just assembling the url, and then if the assembled url is NOT a valid Url type, it assumes the route must be relative and adds the forward slash.

This is no different from the query, how we automatically add the ?, or hash how we always normalize the value to starting with #. "Paths" (when assembled) are expected the start with /, so if the developer omits it, we simply add it for them.

as a side-quest this PR removes complexity from url parts (path, host, query, hash).

• these objects are simple containers now, no toString and no hasValue based on potentially stale values
• logic for cleaning and joining url parts is moved to "withPath", "withHost", etc.

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[pleek91]

Overall this looks good and I like the direction.