Add support for expand

[dickermoshe]

e.g.

class Foo:
  ...

class Bar:
  foo:Foo


bars = [Bar(), Bar(), Bar()]
foos =  Iter(bars).exapnd(lambda x: x.foo) # List of foos
print(foos)
 [Foo(), Foo(), Foo()]

Inspiration from https://api.dart.dev/stable/2.8.2/dart-core/Iterable/expand.html

[MartinBernstorff]

Thanks a lot for the suggestion! By default, I'd like this project to have a consistent API and, somewhat arbitrarily, we're leaning on rust for that (as written in the readme).

If you can find a comparable API on:

• https://doc.rust-lang.org/stable/std/iter/
• https://docs.rs/itertools/latest/itertools/index.html

My impression is that you example is equal to:

Iter(bars).map(lambda x: x.foo)

And the linked example from dart is equal to:

Iter([1,2], [3,4]).flatten() # [1,2,3,4]