feat: add lazy and consumable subtype

[MartinBernstorff]

Currently, to avoid state, the main class copies the iterator on every invocation. This is very expensive memory-wise.

Grabbing inspiration from flupy is likely a good move.

I am most uncertain about how well we can replicate:

• flatten

To decrease memory use, we can re-introduce the state by using generators. However, this creates a lot of non-functional behaviour. To separate this clearly, we might want to introduce ConsumableIter or similar.

[MartinBernstorff]

This could be implemented by overriding the _iter property.