Upgrade SpiderMonkey

[jdm]

Requires servo/mozjs#18 and servo/rust-mozjs#106.

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/2616

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In order to help critic track your changes, please do not make in-place history rewrites (e.g. via git rebase -i or git commit --amend) when updating this pull request.

[jdm]

For the benefit of reviewers:

• Root<T> is Servo's Rooted<T>, and is structurally equivalent so it can be stored in SM's root stacks
• Temporary<T> encapsulates identical behaviour to PersistentRooted<T> (which is duplicated in PersistentRootedObjectElement, structurally equivalent to PersistentRooted<JSObject*>)
• JS<T> acts as Heap<T>
• MutNullableJS<T> is a Heap<T> that implements write barriers

Also take a look at servo/rust-mozjs#106, which does some lower-level integration with SpiderMonkey.