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);