Fix non-idempotent serialization for ./ and ../ paths (fixes #601)

[djc]

No description provided.

[valenting]

I think I didn't handle the code path for the setters 🤔

[bors-servo]

☔ The latest upstream changes (presumably d7dab15) made this pull request unmergeable. Please resolve the merge conflicts.

[annevk]

In the standard this ended up impacting the URL serializer exclusively. It would surprise me a bit of all the parser changes here were needed.

[valenting]

In the standard this ended up impacting the URL serializer exclusively. It would surprise me a bit of all the parser changes here were needed.

rust-url builds the serialization as it parses the input.
For speed mostly. This way serializing is just returning a reference to the serialization which is just one string - no allocations.