cannon: Add basic types for MTCannon

[mbaxter]

Description

Add basic types required for Multithreaded Cannon:

• Add MTState
• Add ThreadContext
• Update StepWitness to reflect upcoming changes

There are a number of TODOs and details left to be figured out that will be addressed in https://github.com/ethereum-optimism/client-pod/issues/903. The purpose of this PR is to provide some basic types to facilitate parallelization of the remaining MTCannon work.

Tests Mirrored a few existing tests from state_test.go into statemt_test.go. These provide some basic validation of MTState (de)serialization and witness hashing.

Metadata

Fixes https://github.com/ethereum-optimism/client-pod/issues/952