* separate block preparation methods
* Split internal sealing from work sealing, add cli option
* replace cli with engine method, simplify
* More docs about sealing types. Bypass work in external txs.
* split requires_reseal, add test and new test miner
* 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.
