Auto merge of #121204 - cuviper:flatten-one-shot, r=the8472

Specialize flattening iterators with only one inner item

For iterators like `Once` and `option::IntoIter` that only ever have a
single item at most, the front and back iterator states in `FlatMap` and
`Flatten` are a waste, as they're always consumed already. We can use
specialization for these types to simplify the iterator methods.

It's a somewhat common pattern to use `flatten()` for options and
results, even recommended by [multiple][1] [clippy][2] [lints][3]. The
implementation is more efficient with `filter_map`, as mentioned in
[clippy#9377], but this new specialization should close some of that
gap for existing code that flattens.

[1]: https://rust-lang.github.io/rust-clippy/master/#filter_map_identity
[2]: https://rust-lang.github.io/rust-clippy/master/#option_filter_map
[3]: https://rust-lang.github.io/rust-clippy/master/#result_filter_map
[clippy#9377]: rust-lang/rust-clippy#9377

Author: bors