RFC: Rename Future to Task in public facing interface

Please don't make too much of this issue. This is mostly to start a conversation.

I spent the week with a bunch of folks who are less sophisticated than our typical users, but who really need something very much like Dask. It was educational.

My sense is that our usual idiom:

```python
future = client.submit(func, *args, **kwargs)
```

... is kinda weird for them. Passing the function as an argument is strange (another proposal for that in https://github.com/dask/distributed/pull/7936). Also, folks mentioned that the term "future" doesn't really make any sense to them. Instead, the term "task" made more sense. This doesn't seem unreasonable. We chose the term future to be consistent with the `concurrent.futures` module, which is mostly just known by devs.

I'll leave the decorator stuff for another conversation, but here I'd like to propose the following cosmetic API change:

```python
>>> task = client.submit(func, *args, **kwrags)
>>> task

>>> task.result()
...
```