style: Tweak the UA style a little bit.
Merge pull request #3499 from pcwalton/ua-fixes

style: Tweak the UA style a little bit.

Reviewed-by: SimonSapin
Add an extra lifetime parameter to TElement / TNode

This isn't used for anything yet, but it's easier to propagate this
change in a separate commit.
Refine a lifetime parameter
Remove the 'static lifetime parameters from TElement methods

This improves the static correctness of layout's use of DOM nodes.

Fixes #2595.