Switch to using ContravariantLifetime in JSRef<'a, T>.

Since ContravariantLifetime doesn't take up any storage space, this
means that JSRef will be a single word. This fixes #2333.
auto merge of #2435 : zwarich/servo/contravariant-lifetime, r=jdm

Since ContravariantLifetime doesn't take up any storage space, this
means that JSRef will be a single word. This fixes #2333.