Add more task types, especially for idle workers

[hai-rise]

pevm/src/lib.rs

Lines 197 to 205 in 1fe6665

	// A scheduled worker task
	// TODO: Add more useful work when there are idle workers like near
	// the end of block execution, while waiting for a huge blocking
	// transaction to resolve, etc.
	#[derive(Debug)]
	enum Task {
	Execution(TxVersion),
	Validation(TxVersion),
	}

Ideas:

• Convert heavy types like from revm::ResultAndState to pevm::PevmTxExecutionResult.
• Collect garbage memory.