solana: Hello World, missing message length check

[a5-pickle]

Objective

Add message size check for both send_message and receive_message. Messages should not exceed the max account size of the Received account. RE: TODO doc.

Because the Received account's max size is 512 bytes, the Hello World program needs to check that the message it deserializes does not exceed this number of bytes.

Consider making the account max size smaller.

Should also add these length checks on the other network Hello World contracts.

Definition of Done

• Added checks to both instructions above
• Tests verifying that checks work (e.g. attempt to send and receive messages that are too large)

[a5-pickle]

We are going to add to the whitepaper a specification of the maximum message size, too.

[a5-pickle]

I changed the objective to make the account's max size for the received payload to 512 from 1028 because if the Wormhole messages exceeds some number of bytes (~860), posting the verified Wormhole message will fail (so we would not be able to redeem this VAA in the Hello World program).