Generate valid Byron blocks

Related: input-output-hk/ouroboros-network#888.

See https://github.com/input-output-hk/ouroboros-network/pull/1320#discussion_r358774453. We have a property test that checks that `verifyBlockIntegrity` detects bitflips in `ByronBlock`. We should also have a test in the other direction: `verifyBlockIntegrity` should return True for all uncorrupted `ByronBlock`s. However, the block generator we have doesn't even generate valid blocks, so checking the signature always fails. This means that the test in the original direction is also not so useful at the moment: the original blocks are already "corrupt".

Note that I have tested `verifyBlockIntegrity` with https://github.com/input-output-hk/cardano-mainnet-mirror/tree/master/epochs already, confirming that it most likely works fine.

To properly solve this, we need a generator that can generate valid Byron blocks.