layout: Rewrite the float context.

This rewrites the float context to avoid dynamic failures resulting from
`.clone()` misuse. It also renames the float context to the simpler
`Floats`. The new version is modeled on WebKit's `FloatingObjects`.
Add check for non-Text children for document.title.(fixes #1742)
auto merge of #1771 : lpy/servo/issue1742, r=Ms2ger

see #1742
Remove no-op implementations from Element and add some spec links.
auto merge of #1774 : Ms2ger/servo/element-noops, r=jdm
layout: Stop racing on the JSManaged borrow flags
auto merge of #1772 : pcwalton/servo/borrow-flags-race, r=jdm

r? @jdm
auto merge of #1770 : pcwalton/servo/rewrite-float-context, r=kmcallister

This rewrites the float context to avoid dynamic failures resulting from
`.clone()` misuse. It also renames the float context to the simpler
`Floats`. The new version is modeled on WebKit's `FloatingObjects`.

r? @kmcallister
Add url getter to Page and fix users of Page url with it.(fixes #1762)
auto merge of #1776 : lpy/servo/issue1762, r=jdm

see #1762
Move Error, Fallible, ErrorResult out of utils.rs (fixes #1749)