Use the newly stored line as the starting line number when
evaluating JS. This ensures that inline scripts will report
errors with meaningful line numbers.
If the last child of a node is a Text child and we are inserting text in that node,
we need to append it to that Text child. Doing that means that
HTMLStyleElement::children_changed gets called less frequently, but that's not a
problem during parsing thanks to the pop hook.
It could be used to have mutable JSVal fields without GC barriers.
With the removal of that trait, MutHeap and MutNullableHeap can respectively
be replaced by MutJS and MutNullableJS.
Free-standing fonctions parse_html and friends are now static methods
on ServoParser, and the HTML and XML tokenizers have been moved to private
submodules.
