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This patch adds: - Load and use a third-party EVM in a shared library, instead of Nimbus EVM. - New option `--evm` to specify which library to load. - The library and this loader conforms to the [EVMC] (https://evmc.ethereum.org/) 9.x specification. Any third-party EVM which is compatible with EVMC version 9.x and supports EVM1 contract code will be accepted. The operating system's shared library format applies. These are `.so*` files on Linux, `.dll` files on Windows and `.dylib` files on Mac. The alternative EVM can be selected in two ways: - Nimbus command line option `--evm:<path>`. - Environment variable `NIMBUS_EVM=<path>`. The reason for an environment variable is this allows all the test programs to run with a third-party EVM as well. Some don't parse command line options. There are some limitations to be aware of: - The third-party EVM must use EVMC version 9.x, no other major version. EVMC 9.x supports EIP-1559 / London fork and older transactions. - Nested `*CALL` and `CREATE*` operations don't use the third-party EVM yet. These call the built-in Nimbus EVM. This mixing of different EVMs between levels is explicitly allowed in specs, so there is no problem doing it. - The third-party EVM doesn't need to support precompiles, because those are nested calls, which use the built-in Nimbus EVM. - Third-party EVMs execute contracts correctly, but fail the final `rootHash` match. The reason is that some account state changes, which are correct, are currently inside the Nimbus EVM and need to be moved to EVMC host logic. *This is a known work in progress*. The EVM execution itself is fine. Test results using "evmone" third-party EVM: - [evmone](https://github.com/ethereum/evmone) has been tested. Only on Linux but it "should" work on Windows and Mac equally well. - [Version 0.8.1](https://github.com/ethereum/evmone/releases/tag/v0.8.1) was used because it is compatible with EVMC 9.x, which is required for the EIP-1559 / London fork, which Nimbus supports. Version 0.8.0 could be used but it looks like an important bug was fixed in 0.8.1. - evmone runs fine and the trace output looks good. The calls and arguments are the same as the built-in Nimbus EVM for tests that have been checked manually, except evmone skips some calls that can be safely skipped. - The final `rootHash` is incorrect, due to the *work in progress* mentioned above which is not part of the evmone execution. Due to this, it's possible to try evmone and verify expected behaviours, which also validates our own EVMC implementation, but it can't be used as a full substitute yet. Signed-off-by: Jamie Lokier <jamie@shareable.org>