Lint unnecessary vec.extend(slice.to_vec())

[borngraced]

Adds a check under unnecessary_to_owned that catches vec.extend(slice.to_vec()),
vec.extend(slice.to_owned()), and vec.extend(vec_ref.clone()) where the argument is a
needless heap allocation and suggests vec.extend_from_slice(...) instead...closes #16725

changelog: [unnecessary_to_owned]: lint unnecessary vec.extend(slice.to_vec())

[rustbot]

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[github-actions]

Lintcheck changes for 072fa0c

Lint	Added	Removed	Changed
clippy::unnecessary_to_owned	1	0	0

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