chore: change the header field of `types.Block` to be a pointer `*Header`

The header type is quite big (184 bytes) so it should be passed around as a pointer, not a value, in the block type.

The alternative would be to handle `types.Block` as a pointer and change `NewBlock` to return a pointer to a block, but this might lead to more deltas and is less foolproof than the first approach which enforces the enhancement for block values and pointers to block.