Fix the out-of-bound failure in stylesheet iterator.

Rewrite the iterator as internal instead of external.
This is much easier for recursive tree traversal.
Fix unused import warnings.
auto merge of #1137 : SimonSapin/servo/fix-stylesheet-iterator, r=metajack
auto merge of #1140 : SimonSapin/servo/unused-imports, r=metajack
Optimize reflow by changing enums to traits and inlining more
auto merge of #1132 : pcwalton/servo/optimize-reflow, r=larsbergstrom

r? @metajack
submodules: Update submodules.
submodule: Update rust-http to a fixed version
Add rainbow perf test case
Move special cases of CSS 'display' the property to computed value, per spec.
Use Au::scale_by instead of duplicating it.
Fix selector parsing.
Move CSS21 pseudo-elements parsing out of parse_simple_pseudo_class
Implementation of pseudo class 'first-child'.
auto merge of #1143 : SimonSapin/servo/selector-parsing+first-child, r=SimonSapin

Fixes #1133, fixes #1134.

This is a rebase and clean-up of these two pull requests that have been reviewed individually. Additionally, 5c59699 disables parsing of `display: list-item` which is not supported yet and may have been the cause of the hangs when testing #1134.
Remove some at-pointers that are not actually needed. (Selector matching)
auto merge of #1146 : SimonSapin/servo/less-at-pointers, r=jdm