feat: increase simulation likelihood of low probability events

[bhgomes]

This PR adds some simulation hooks for sampling events that are unlikely in the current simulation:

• Adds SelfTransfer simulation action for an actor to perform a private transfer to itself
  • unlikely to happen in the normal simulation because an actor choosing its own receiving key from the pool goes down with time
• Adds FlushToPublic simulation action to reclaim all private assets
  • unlikely to happen in the normal simulation because the random sampling for asset withdrawal is uniform over the current balance, so it is unlikely to hit the maximum value
• Adds MintZero, PrivateTransferZero, and ReclaimZero environments for testing
  • for the same reason FlushToPublic is unlikely
• Upgrades GenerateReceivingKeys to sample a number of keys to generate from a Poisson distribution with λ = 1
  • want to increase the number of receiving keys to stress-test signer storage maps