Allow eager scheduler to accept non-eager thunks

And allow lazy schedulers to accept eager thunks? Without cache-sharing between schedulers, this will be rather inefficient, but could potentially be helpful for code which already has `Thunk`s describing a computation (such as when provided by a library).

The alternative is the status quo: `collect` `Thunk` arguments before passing them to the eager scheduler, and `fetch` `EagerThunk` arguments before passing them to a lazy scheduler.