Add `.flat_map()`

[epage]

Implementing .flat_map() is a bit of a journey because I wanted to implement this like the stdlib (even ordering them the same in Ext as Iterator), which meant adding IntoGenerator and modifying .flatten() to take it. from_iter() was from a failed experiment towards this.

All steps in this journey are broken out into individual commits for perusing convenience.

Potential next steps:

• Add more IntoGenerator impls
• Add a FlatMap newtype.
• Maybe even hand-implement FlatMap

[epage]

All set.

[AndWass]

Thanks! Very nice work! Much appreciated!