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Auto merge of #108671 - Coca162:option_reference_flattening, r=dtolnay
Implement flatten for `Option<&Option<T>>` and `Option<&mut Option<T>>`
Adds `.flatten_ref()` to get a immutable reference of a nested option and `.flatten_mut()` for mutable references.
ACP: https://github.com/rust-lang/libs-team/issues/186
Tracking Issue: rust-lang/rust#149221
