Support for andThen(Function1)?

Hi,

Currently, if a `Task` needs to be executed after a task, sequentially, then `andThen(Task task)` needs to be used.
This introduces a few issues where if code is like `andThen(someTaskReturningFunction())` then `someTaskReturningFunction()` will get executed though the `Task` returned by it may not be run.
Should a `andThen(Function1 func)` be supported to allow sequential execution of a task after a parent task by also ensuring the task supplying argument is not invoked.
This will be close to `withSideEffect` but sometimes, `andThen` is more suited for the use case.

Start by locating Task and the existing andThen(Task task) and withSideEffect APIs. Compare their execution behavior with the proposed andThen(Function1) overload; done means the function is not invoked until the parent task completes and its returned task can then execute sequentially.

Written by the indexing model from the issue text.