Make the block header struct's internals private

[nipunn1313]

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.

This begins work on #1267

[nipunn1313]

Bam Pull Request number 2000!

[rphmeier]

should be able to just do if header.difficulty() < &min_difficulty

[rphmeier]

a lot of the clones in comparisons can be avoided but LLVM will probably optimize out extraneous copying. H256 probably shouldn't be Copy as it isn't that small imo.

[coveralls]

Coverage increased (+0.03%) to 86.366% when pulling 18c36f7 on nipunn1313:privateheader into 2d883c4 on ethcore:master.

[nipunn1313]

Great point. I went through and found places where we could eliminate clone() by comparing references. Unfortunately, it seems like you can't compare a &H256 with an H256 directly because of rust-lang/rfcs#1332, but by making both sides references, it compiles nicely as @rphmeier suggested.

I also went through and fixed up the tests and tried to minimize explicit cloning. Any time + or * is used for math, I had to do some cloning. I think the llvm compiler will do a good job of optimizing those situations.