Add two utils to Lotus-shed

[arajasek]

• The first is to help with base64 inputs
• The second can be used to validate signed votes for FILPolls, so that users can be confident that they have not been tricked into signing something other than a ballot. To use it for the FIP-0014 poll, run

lotus-shed signatures verify-vote 14 <signingAddress> <signature (hex string)>

Example:

lotus-shed signatures verify-vote 14 f1vstk2dncjlimtmbcmu7tjxwekaurhvvma3joqea 01c109f5c39d6792e991a6f8d74f081f98850bdcdb1c0432608e9d30dcaeaf030a5e9c7101bb12ffa7a4c0b311308462763b928edc7828d6563e1e2b8562aa3d8b00
valid vote for rejecting FIP-0014