implement block_set and block_dump

[hannesm]

also, revise block_add to optionally take data. alternative to #82

//cc @dinosaure would this suit your needs?

[hannesm]

this extends the wire protocol with some backwards compatibility:

• a new server will understand the old client (`C2 of int is Block_add (size, None))
• a new client will issue `C4 for Block_add, which an old server will reject (but here, the client can be downgraded)