I don't need this per se, but it seems like a footgun for the methods to
return incorrect information depending on the build type. I don't see anywhere
where the overhead would be at all significant.
MozReview-Commit-ID: G1qyUFhI0aB
This patch introduces infrastructure for the rule tree, and constructs it.
We don't use it yet, nor have good heuristics for GC'ing it, but this should not
happen anymore once we store the rule node reference in the node.
I haven't messed up with memory orders because I want to do a try run with it,
then mess with them.
Take down the ApplicableDeclarationsCache, use the rule tree for doing the cascade.
