lazy mable and fable

[earowang]

I've prototyped the idea of rolling tsibble https://github.com/tidyverts/tsibble/tree/137/slide-by

I like the collect() idea, only evaluating when user asks for it. model() and forecast() may only hold the queries. When collect()ing, evaluation takes place. accuracy() eventually realises the lazy evaluation.

[mitchelloharawild]

Sounds interesting, would be nice to see a demo of the slide by functionality.