This introduces a new BlockHeader helper class which Block and
MutableBlock can both produce, which helps a lot of methods to
be ported to no longer be dependent on us using a MutableBlock
object, which is too costly to use when we have no intention
to alter the block.
This avoids a naming conflict with a p2p class BlockHeader.
Notice that the block data structures are mostly still private API, they
are in the utils dir but headers are not installed, nobody has needed
them so far.
tomFlowee