Checking block validity

[naterush]

Issue

All messages are currently valid in the repo. Validator's might equivocate, non-validators might send messages, the bit might be 2 instead of 0/1, etc. We would like validators to check validity of messages upon arrival (and maybe perform actions if invalid message).

Proposed Implementation

• define valid to check validity of specific types of messages
• write tests to ensure invalid blocks do not affect validator's view/estimates
• create function for validator to call for actions to be taken if invalid message (can just pass if nothing)

[naterush]

Upon arrival (aka, no longer pending!)