feat(format_push_string): give a (possibly incomplete) suggestion

[ada4a]

Resolves #8824

Supersedes #15828 -- I wasn't able to get TyCtxt::in_scope_traits to work, so here's a more basic implementation that just adds a note about the possible need to import std::fmt::Write.

changelog: [format_push_string]: add a suggestion

[rustbot]

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[github-actions]

Lintcheck changes for 5ea08dd

Lint	Added	Removed	Changed
clippy::format_push_string	0	0	29

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