Refactor/core

[Autoparallel]

Give an overview of the tasks completed
We worked to bump to revm v4.0.0 and revm-primitives v2.0.0. This was also a large scale refactor of arbiter-core. Goal was to reduce complexity and any components that weren't really needed.

• Consolidated error types into ArbiterCoreError enum
• Improved error handling and succinctness
• Renamed RevmMiddleware to ArbiterMiddleware
• Moved src/tests/ to tests/ as this is standard
• Removed a handful of shallow modules
• Reorganized src/ layout
• Consolidated and remapped imports (e.g. ethers::types::U256 as eU256 everywhere)
• Homogenized middleware/client inputs into all ethers types. Casting into alloy/revm types only internally.
• Probably some other random stuff

Link to issue(s) that this PR closes

• Closes feat(arbiter-core / arbiter-engine): revert handling #823
• Closes deps(revm): bump to 4.0.0 #859

[0xJepsen]

lgtm