message: Add CHANGELOG entry for read length limit
diff: internally).
  - `call_{init,handle,query}` and the `cosmwasm_vm::testing` wrappers return
  standard `Result` types now, eg. `Result<HandleResponse, ApiError>`.
  +- Add length limit when reading memory from the instance to protect against
  + malicious contracts creating overly large `Region`s.
  ## 0.7.2 (2020-03-23)
  