Change representation of CFI rules

This patch changes CFI rules to use a simpler representation that covers most of
what we hit in practice, skipping Postfix expressions (but leaving them as a
fallback path).

This is the first of a somewhat-invasive set of patches, all by Julian Seward.
He has patches to replace the unwinding of Firefox's built-in profiler with
Breakpad's unwinder, but the performance wasn't quite where we wanted it to be
for real-time profiling (especially on mobile). He did some profiling of stack
unwinding, and found that we spent a lot of time in the PostfixEvaluator,
primarily doing string comparisons. Since it uses strings as keys and values in
a bunch of maps this is not terribly surprising. We brainstormed some and came
up with the solution presented here. There are a few followup patches that
further optimize things.