Allow using taskFor at assignment

[jamescdavis]

This adjusts the assertion on the argument passed to taskFor such that it can be used at assignment, e.g.

@task
myTask = taskFor(function*() {
  yield 1;
  return 2;
});

[chancancode]

This doesn't work for encapsulated tasks. Unfortunately, if we want this to work I think we probably just have to give up on having an assertion at all. The original intent for this is to guard the fact that, despite allowed by the types, passing a normal generator or async function here does not magically turn it into a task, and this really only works if you have the decorator pre-applied. Now if we are purposefully allowing that, I think we'll just have to remove the assertion. We could update it at check specifically for the kind of functions or POJOs that are allowed by the decorator, but at that point we would just be duplicating what TypeScript already checked for.

[jamescdavis]

It actually works just fine because an encapsulated task descriptor is an object with a function named perform. I've added a test proving so. 😄

[chancancode]

I think I am okay with this, but can you add tests for encapsulated task in assignment usage, as well as mentioning this in the README as an alternative?

[jamescdavis]

@chancancode as a first step: #3

If you want to get that in first, I'll rebase this.

[jamescdavis]

Ok, I've added the encapsulated task test and documented this usage in the README.

Once chancancode/ember-concurrency-async#7 is released, I'll go back and update these docs to add:

@task myTask = taskFor(async () => {
  await this.something;
  return 'done!';
}

to show how using an async arrow function removes the need to assert the type of this.

[chancancode]

I think #3 is almost good to merge other than fixing one import issue, let's merge that and rebase this on top

[chancancode]

Merged #3, this will need a rebase

[jamescdavis]

@chancancode rebased

[chancancode]

Thank you! Also you can merge/push/release now. 😛 To release, just run yarn version locally and push the tag/commit and CI will take care of the rest (don't forget to push the commit to the branch too).