Deprecate simple.NewOperator and update examples (#565)

Added deprecation comment to `simple.NewOperator` and `simple.Operator`.
`examples/operator/simple` was still using `simple.Operator`, so the
code has been updated to use `simple.App` and `operator.Runner`.
`examples/operator/opinionated` was building an operator from scratch,
which is no longer advised, so the example has been removed.

Author: IfSentient