New mining framework.

Fixes #756.

ethcore/src/ethereum/ethash.rs

@@ -101,7 +101,7 @@ impl Engine for Ethash {
 	/// This assumes that all uncles are valid uncles (i.e. of at least one generation before the current).
 	fn on_close_block(&self, block: &mut ExecutedBlock) {
 		let reward = self.spec().engine_params.get("blockReward").map_or(U256::from(0u64), |a| decode(&a));
-		let fields = block.fields();
+		let fields = block.fields_mut();
 
 		// Bestow block reward
 		fields.state.add_balance(&fields.header.author, &(reward + reward / U256::from(32) * U256::from(fields.uncles.len())));