Make `Data.Function#apply` point-free

[triallax]

Description of the change

Just made apply point-free.

I know this sounds like a weird change to make a full PR for (and it is), but it's just been nagging my mind since forever, so I thought I'd just go ahead and do it. Feel free to reject this PR too; that works for me, as the nagging has already stopped.

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Checklist:

• [ ] Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)") (does this change deserve being in the changelog?)
• [ ] Linked any existing issues or proposals that this pull request should close
• [ ] Updated or added relevant documentation
• [ ] Added a test for the contribution (if applicable)