Add a len field to track the size of the window.

[silvestrpredko]

Currently, the iterator of the window produces samples indefinitely. This is not intuitive at first glance. Also, if I just want to work with samples in a plain for each, I need to remember to call take a fixed number of samples first; otherwise, it will loop indefinitely.

[PanieriLorenzo]

I understand that it seems unintuitive for Window to be an infinite iterator, however the reason is that it is a signal. Signals are modelled as infinite iterators, that's just the way the crate is designed, and it also is how signals are usually modelled in DSP theory.

The behavior you want is actually already implemented in Windowed, which according to the docs "Returns None once the Window has been exhausted."

Windower yields Windowed, not Window, so your test is actually not testing the change you made.

While this change makes sense, it is a breaking change, and it also diverges from how signals are represented in the rest of the crate. Signals in dasp_signal are infinite iterators, this is just how the crate is designed.