require miners to run full nodes

[zack-bitcoin]

It may be useful to prevent SPV mining.
The worry is that miners will mine blocks on top of a header without taking the time to verify that the block that header was built from is valid.
They may do this if, for example, the block is unavailable.

Headers currently need to include a merkel root of the txs from their block.
We could also include a Merkel root of the txs from the previous block, but they should all be salted with like a single 0 byte each.
That way, it is impossible to mine a valid block without knowing the txs from the previous block.