message: Update CHANGELOG with Context
diff: favour of `StdError::generic_err` and friends.
  - Implement `From<&[u8; $N]> for Binary` and `From<[u8; $N]> for Binary` for all
  `$N <= 32`.
  +- Add `Context` object that can be used to build Init/Handle/Migrate response
  + via `emit`, `send_action`, `set_data` and then convert to the proper type
  + via `try_into`. Option to simplify response construction.
  **cosmwasm-vm**
  