Although you can technically get at this value via the MaxPayloadLength
function on a block, it is less overhead for any consumers that need to
know the value to simply export it directly.
This commit changes MsgBlock to enforce a 1MB max payload per the spec.
Previously it was only limited to the max overall message size. While
here, also enforce max payloads per message type (instead of only the max
overall message payload) when writing messages.
The functions for generating transaction and block hashes contained a few
error checks for conditions which could never fail without run-time
panics. This commit removes those superfluous checks and adds explanatory
comments.
