feat: precompilegen Solidity interface -> stateful precompile

[ARR4N]

Why this should be merged

Note that this is not a full replacement for ava-labs/subnet-evm/cmd/precompilegen. The original conflates the general concept of stateful precompiles with Ava-specific functionality (e.g. implementation configuration, module registration, etc.) while this PR is purely a mechanism for creating precompiles that conform to Solidity interfaces. In addition to the benefits I outline below, there is a case for separation of concerns.

The main benefits of this approach are that:

1. The implementation is separate from the wrapper that converts it into a vm.PrecompiledContract. The subnet-evm generator leaves place for custom code to be inserted, making it impossible to update the boilerplate without overwriting customisation.
2. Only the Go interface and the precompile constructor are exported, encapsulating all helper functionality to make generated packages simpler to understand.
3. Fallback functions behave in the same way as expected by Solidity developers, accepting and returning a raw byte array.
4. TODO re libevm integration

How this works

Code generation is analogous to gRPC server stubs:

	gRPC	precompilegen
Input	proto	Solidity -> solc ABI output
Generated	type FooServer interface	type Contract interface
Construction	func RegisterFooServer(..., FooServer)	func New(Contract) vm.PrecompiledContract

interface IPrecompile {
    function Foo(uint256) external returns (bool);
}

becomes

type Contract interface {
    Fallback(vm.PrecompileEnvironment, []byte) ([]byte, error)
    Foo(vm.PrecompileEnvironment, *big.Int) (bool, error)
}

func New(Contract) vm.PrecompiledContract { ... }

which is used for the return values of params.RulesHooks.PrecompileOverride.

Tip

See libevm/precompilegen/testprecompile/generated.go for output and libevm/precompilegen/testprecompile/generated_test.go for an example implementation.

To-do

• Remove explicit gas return argument in favour of contract.UseGas() bool equivalent on vm.PrecompileEnvironment (refactor: PrecompileEnvironment.{,Use,Refund}Gas() in lieu of args #73).
• Handle payable and view/pure implications.

How this was tested

Integration test exercising Solidity-based test via a simulated.Backend.