The reason I didn't do this in thunk.el is that thunks are just bare
lambdas, so there is no way to get rid it "from the inside", as it were.
Whereas for streams, it's just a matter of list editing.

Some additional things that I thought of changing, but I didn't yet:

- stream--identifier vs just using '--stream-- directly, I don't see
  what's the benefit of indirection here.

- stream-make should use cons instead of list (or maybe a struct?).

- stream-empty should just be a constant.