op-deployer: Add design doc on using Forge (#325)

Author: mslipper

protocol/op-deployer-forge-engine.md

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+# Use Forge as the EVM for OP Deployer
+
+## Summary
+
+We propose switching out the Go-based EVM inside OP Deployer for a wrapper around Forge. This would reduce our
+maintenance burden and allow us to take advantage of new Forge features without requiring us to reimplement them.
+
+## Problem Statement + Context
+
+OP Deployer was developed on top of a Golang-based EVM. This allows OP Deployer to execute Forge scripts and
+simulate transactions prior to executing them like Forge does. Now that OP Deployer has been in production for over
+a year, we're running into several issues with this approach:
+
+1. **Difficulty debugging.** Rather than outputting a trace of the error, OP Deployer simply panics with
+   "revision ID 1 cannot be reverted." Users must then parse through the logs to determine the cause of the error.
+2. **Incomplete featureset compared to Forge.** OP Deployer is often compared to Forge, since both deploy contracts.
+   Therefore, users perceive OP Deployer as incomplete from a feature perspective. For example, OP Deployer does not
+   support verifying all the contracts it can deploy, and the contracts it _does_ support can only be verified on
+   Etherscan.
+3. **Maintenance burden.** The Golang EVM is a significant maintenance burden. Every new cheatcode in Forge must be
+   reimplemented in Go. Additionally, we must maintain a "bridge" to allow Golang to call into Solidity scripts.
+   This is a lot of work that provides no user benefit since the Golang EVM is simply a backend that allows contracts to
+   be deployed. This is table stakes for a deployment tool, and is much better solved in Forge.
+
+We originally adopted the Golang solution because we wanted to avoid adding a dependency on Forge. However, there
+are many ways around this that do not incur the maintenance burdens described above.
+
+## Proposed Solution
+
+We propose switching all contract calls in OP Deployer to use Forge instead of the Golang EVM. OP Deployer would
+essentially become a thin, stateful wrapper around Forge. Think of it like an installer for our contracts. We've
+outlined some implementation details of this approach below.
+
+### Getting Forge
+
+OP Deployer will autodetect Forge based on the following heuristics:
+
+1. If Forge is already on the user's machine, and it is a supported version, use it.
+2. Otherwise, download a pinned version of Forge and use that.
+
+### Calling Forge
+
+For now, OP Deployer will simply call the Forge binary as a CLI tool. To communicate with Forge, OP Deployer will
+parse Forge's standard output stream. This will require updating some of our smart contracts to output JSON. We may
+adopt solutions like calling Forge as a library via FFI, but that is currently out of scope.
+
+Forge's output will otherwise be streamed directly to the user's terminal so that they can see the progress of the
+deployment.
+
+## Impact on Developer Experience
+
+This change should make maintaining OP Deployer much easier:
+
+1. The Platforms team would no longer need to maintain the Golang EVM.
+2. Changes to Forge can be immediately adopted.
+3. Smart contract devs don't need to worry about how their contracts will be called from Golang anymore.
+4. Error messages and contract deployment output will be much better.
+
+Users running OP Deployer in a container will either need to use a container that contains Forge, or allow that 
+container to download the Forge binary. We can provide a prebuilt container with Forge installed to avoid this issue.
+
+## Alternatives Considered
+
+### Continue Maintaining the Golang EVM
+
+This is our default path. However, we're getting asked to build things like Blockscout contract verification, which 
+would be entirely duplicative of what Forge already does really well.
+
+## Risks & Uncertainties
+
+None really. This is a pretty straightforward change that reduces risk overall.