URLs are parsed and produce records

No platinum yet, but this changes a lot and not all changes were done
manually so review is appreciated.

This commit changes the following:

* Parsed URLs are now URL records.
* Resolve a URL is now parse a URL.
* Resolve a URL returns a resulting URL string and a resulting URL
  record now.
* Resolve a URL takes a document or environment settings object now
  rather than an element or base URL.

This should resolve this bug and issue:

* https://www.w3.org/Bugs/Public/show_bug.cgi?id=26402
* #532

There is room for future cleanup:

* Stop using the “resulting URL string” and “resulting URL record”
  concepts in most scenarios. Assigning the result to a variable is
  clearer.
* Figure out if more places ought to invoke the URL parser directly (in
  particular standalone APIs where we want to use UTF-8 as encoding
  rather than defaulting.)