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.