if we define a TraceableAllocator at all times and use that in places
that want maybe-traceable allocation we can simplify things a lot. we
also unconditionally allocate cache root pointers for Value and Env-1
caches, even though we don't need them without gc (they're so cheap).
defaulting to `std::allocator` without gc recovers previous behavior.
Change-Id: I236da8c3b0669b40cdfe355ec3ec4e764d096074
eldritch horrors
2e5780ebc8