* cannon: Rename StepWitness.MemProofs to ProofData

* cannon: Add MTState type (in progress)

* cannon: Tweak MtState tests to cover more ground

* cannon: Add test for MTState.UpdateCurrentThread()

* cannon: Use constants for byte size vars, set byte slice capacities

* cannon: Add StepsSinceLastContextSwitch field

* cannon: Rename witness offset constants

* cannon: Rename ThreadContext to ThreadState

* cannon: Panic on unimplemented method calls

* cannon: Compute thread stack roots lazily

* cannon: Push initial thread to left stack