Allow eager scheduler to accept non-eager thunks

[jpsamaroo]

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 Thunks 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.