WIP: Hello Shard! Intershard communication example app

The relevant commit is 5941750da4d17e7b94d000173d1991b524a40801

Far from elegant and very rudimentary first version that can for sure benefit from many improvements.

Some of the current limitations are:

The client sends messages to a gateway shard, meanwhile the other shard only receives masked messages from this gateway shard.
The complete message transmission, from client to gateway shard to "receiving" shard all happens within an epoch.
The generation of preprocessing elements for the intershard transfers is "fake", as it is done at start up time as though a trusted dealer would distribute field elements (masks shares) to each server.
The contract does not control access to the queue of intershard messages thus allowing "un-authorized" reads.

From the above limitations, things to improve are roughly:

[ ] The message transmission flow can go from client to any shard, and the first receiving shard will forward it to the other.
[ ] Not sure, but it seems to me that the "complete" message transmission (client -> shard 1 -> shard 2 does not need to be all happening within one epoch. That is, the shard that produces messages and the shard that consumes messages do not need to operate at the same rate, since the producer queues up messages and the consumer consumes the messages from the queue; they can be operating at different speeds.
[ ] Implement randousha-based generate_intershard_masks() function.
[ ] Tidy up the access control using a SecretCellArray data structure in the contract.

initc3 / HoneyBadgerMPC