script: Inline some hot functions used in flow constructions.

9% improvement in style recalc on the rainbow page.
Added refs to Element's DOM spec

This is a sub-task for #1660.
Added refs to live HTMLCollections issue
Fixed FIXME message as pointed out by jdm
Rename starts_*_flow to is_*_flow
Change Text, Comment and ProcessingInstructions parent name from element to characterdata

All other node's parent type names reflect the actual type of the parent. This change extends that convention to the indicated nodes.

closes #1594
auto merge of #1702 : quandrum/servo/element_to_characterdata, r=jdm

... to characterdata

All other node's parent type names reflect the actual type of the parent. This change extends that convention to the indicated nodes.

closes #1594
Simplify string construction in InterfaceObjectToString.
Use DOMString in a few more places in utils.rs.
Use DOMString instead of ~str in a few places.
Use eq() and eq_slice() less, and map_default() more.
Use push_str instead of operator + for efficiency.
Make some selector matching code a little more consistent.
auto merge of #1703 : Ms2ger/servo/strings-cleanup, r=jdm
Change <em> and <strong> to be HTMLElement (fixes #1704)
auto merge of #1705 : lpy/servo/issue1704, r=Ms2ger

see #1704
Refactor cascade() and fix cascade_with_cached_declarations()

* Expand the apply() and apply_cached() templates.
  Their two invocations each were different enough
  that this improves readability IMO.
* Create computed::Context from inherited and cascaded values
  rather than computed value, as much as possible.
* Centralize this creation rather than making it per-property,
  making 'needed_for_context' not needed anymore.
* Pass a context to cascade_with_cached_declarations() rather than
  duplicate the creation code.
auto merge of #1667 : brunoabinader/servo/dom-element-refs, r=jdm

This is a sub-task for #1660.
auto merge of #1602 : pcwalton/servo/more-inlines, r=jdm

9% improvement in style recalc on the rainbow page.

r? @jdm
Ensure id attributes on elements only affect elements present in a document.