Ensure correct typespecs of Events and idempotence of decode.

[cognivore]

Why?

Currently, vents seem to have wrong typespecs, they only encode the target data type, ignoring the incoming raw data type we decode. This is a bug.

Furthermore, when we decode(decode(x)), we crash. Which is suboptimal, so decode should be made idempotent unless there is something that guarantees that decoded events will never be decoded. In that case, in line with the principle of fail fast, fail loud, decode should NOT be made idempotent, but still should be at least guarded.

How?

• Take a look at this commit.
• Assess if decode should be idempotent or if our system guarantees that a decoded event shall never be decoded.
• If a decoded event shall never be decoded, remove the second head of decode function in this commit, but leave the pattern match.
• Replicate these changes across all the events in the system.

[cognivore]

1f17992