Remove the holderType return value of getJSToNativeConversionTemplate.

It is always None.
auto merge of #2231 : Ms2ger/servo/remove-holderType, r=jdm

It is always None.
Increase codegen test coverage.
Implement FromJSValConvertible for union types.
Use FromJSValConvertible's nullable conversion code.
This batch of changes upgrades Servo to work with the Rust upgrade as of
April 10, 2014. The main changes are to privacy, to work around the
issues with incorrect bounds on the libstd `Arc<Mutex<T>>`, and the
various API changes strewn throughout the libraries.
auto merge of #2238 : mozilla/servo/rustup_20140410c, r=Ms2ger

r? @metajack 

Note that all pending submodule PRs must be landed before this should be given r+.
auto merge of #2237 : Ms2ger/servo/union-fromjsvalconvertible, r=jdm

With bonus codegen tests.
Make ClientRectList::rects a Vec.
Make Document::idmap store a Vec.
Make Element::attrs a Vec.
Make EventTarget::handlers store a Vec.
Make static HTMLCollections use Vec.
Make TreeIterator::nodes a Vec.
Make simple NodeLists use Vec.
fixup! Make simple NodeLists use Vec.
Remove unused support for passing extra arguments for timers.
Make ScriptTask::mouse_over_targets use Vec.
Make PageTree::inner and PageTreeIterator::stack a Vec.
Make open_elements in htmlserializer.rs use Vec.