Reliable user-space stack traces with SFrame

Just a note: the SFrame format is not based on ORC at all (it has rather different goals: simplicity of reading is there, but compactness is valued higher than ORC, plus of course it's targetting representing stacks for all of userspace rather than purely the kernel). The implementation happens to use the same in-kernel API, but that's because it has to: that's the API the in-kernel users are using.