chore: gas optimizations on terabethia contract

[b0xtch]

Once simpleStorage().messages[msgHash] is assigned to 1. Consider setting simpleStorage().messages[msgHash] to zero to show the message has been consumed on the L2.

-Gas optimizations within sendMessage function w/ ~500 gas savings in total:
--cache simpleStorage().nonce to avoid performing multiple SLOADS

--use unchecked math when incrementing simpleStorage().nonce, overflow unrealistic. if this contract were called 100 trillion times a day for 100 years we would still not be close to overflow. overflow can and should be monitored outside of the contract.

--don't increment simpleStorage().messages[msgHash]. instead, assign to 1 to save gas. There's never a situation where this value is anything other than 1 or 0 because the nonce is used when encoding the message hash. The message hash will never be repeated.

• testing
• deploy