Make the block header struct's internals private (#2000)

* Make the block header struct's internals private

Currently, this involves a lot of explicit cloning, but we
could migrate the return types of the get_* functions to
be copies rather than references since they are mostly copy
types anyway.

I opted to eliminate the constructor in favor of using
Default::default() plus calling a bunch of setters. This
is similar to the model that a Google Protobuf client uses
and I think it looks fine.

* Drop some unnecessary cloning by comparing references

* Fix compiler errors from callsites in tests.

ethcore/src/blockchain/blockchain.rs

@@ -1434,7 +1434,7 @@ mod tests {
 		let mut block_header = bc.block_header(&best_hash);
 
 		while !block_header.is_none() {
-			block_header = bc.block_header(&block_header.unwrap().parent_hash);
+			block_header = bc.block_header(&block_header.unwrap().parent_hash());
 		}
 		assert!(bc.cache_size().blocks > 1024 * 1024);
 

ethcore/src/blockchain/generator/block.rs

@@ -44,21 +44,22 @@ impl Encodable for Block {
 
 impl Forkable for Block {
 	fn fork(mut self, fork_number: usize) -> Self where Self: Sized {
-		self.header.difficulty = self.header.difficulty - U256::from(fork_number);
+		let difficulty = self.header.difficulty().clone() - U256::from(fork_number);
+		self.header.set_difficulty(difficulty);
 		self
 	}
 }
 
 impl WithBloom for Block {
 	fn with_bloom(mut self, bloom: H2048) -> Self where Self: Sized {
-		self.header.log_bloom = bloom;
+		self.header.set_log_bloom(bloom);
 		self
 	}
 }
 
 impl CompleteBlock for Block {
 	fn complete(mut self, parent_hash: H256) -> Bytes {
-		self.header.parent_hash = parent_hash;
+		self.header.set_parent_hash(parent_hash);
 		encode(&self).to_vec()
 	}
 }

ethcore/src/blockchain/generator/generator.rs

@@ -73,8 +73,8 @@ pub struct ChainGenerator {
 impl ChainGenerator {
 	fn prepare_block(&self) -> Block {
 		let mut block = Block::default();
-		block.header.number = self.number;
-		block.header.difficulty = self.difficulty;
+		block.header.set_number(self.number);
+		block.header.set_difficulty(self.difficulty);
 		block
 	}
 }