Move stack roots into Root types instead of RootCollection, removing the aribtrary 10 roots per stack frame restriction.
s/Unrooted/Temporary/g
Root Temporary values for the duration of their lifetime.
Store per-ScriptTask RootCollection in TLS and use that in favour of per-frame collections.
Remove all root collections.
Address review comments.
Make dictionaries contain Root<T> values instead of JS<T>, ensuring that they will not be collected while the dictionary is alive.
Fix up the virtual method unsoundness.
Address review comments.
auto merge of #2101 : jdm/servo/newroot_rebase, r=Ms2ger

As described in #1764, this strategy uses the following properties:
* DOM members are `JS<T>` types. These cannot be used with being explicitly rooted, but they are required for compiler-derived trace hooks.
* Methods that take DOM type arguments receive `&[mut] JSRef<T>`. These are rooted value references that are cloneable but cannot escape.
* Methods that return DOM values use `Unrooted<T>`. These are values that may or may not be rooted elsewhere, but callers must root them in order to interact with them in any way. One unsoundness hole exists - `Unrooted` values must be rooted ASAP, or there exists the danger that JSAPI calls could be made that could cause the underlying JS value to be GCed.
* All methods are implemented on `JSRef<T>`, enforcing the requirement that all DOM values are rooted for the duration of a method call (with a few exceptions for layout-related code, which cannot root values and therefore interacts with `JS<T>` and `&T` values - this is safe under the assumption that layout code interacts with DOM nodes that are in the tree, therefore rooted, and does not run concurrently with content code)
Replace all ~"" with "".to_owned().
auto merge of #2311 : Ms2ger/servo/empty-owned, r=jdm
Replace most ~"string"s with "string".to_owned().
Reorder some comparisons to avoid allocating strings for them.
Call find_equiv to avoid allocation strings.
auto merge of #2312 : Ms2ger/servo/owned-strings, r=SimonSapin
Use str.to_owned() in tests.
~[] to Vec in FontStyle and FontGroup
~[] to Vec in style/media_quieres.rs, selectors.rs, stylesheets.rs
~[] to Vec in main/compositing and associated files