message: Update wording in CHANGELOG
diff: `cosmwasm_std`, so we can return cosmwasm-vm specific `Result` (only used
  internally).
  - `call_{init,handle,query}` and the `cosmwasm_vm::testing` wrappers return
  - eg. `Result<HandleResponse, ApiError>` rather than
  - `ApiResult<HandleResponse, ApiError>`. This allows much closer usage to the
  - unit tests which return `Result<HandleResponse, Error>`.
  + standard `Result` types now, eg. `Result<HandleResponse, ApiError>`.
  ## 0.7.2 (2020-03-23)
  