Tracking issue for std::path::Path::ancestors

[teiesti]

Tracking issue for #48420, std::path::Path::ancestors and std::path::Ancestors.

Steps:

• Implementation
• Stabilization PR (Stabilize std::path::Path::ancestors #50894)

Unresolved questions:

• What is a good name? ancestors? parents?

[teiesti]

cc @BurntSushi