[consensus] impl conversions from/to prost proto and Block
[consensus] impl conversions from/to prost proto and Block

Closes: #1210
Approved by: delegate

aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core