This adds a "Convert Into to From" assist, useful since clippy has
recently started adding lints on every `Into`.
It covers converting the signature, and converting any `self`/`Self`
references within the body to the correct types.
It does assume that every instance of `Into` can be converted to a
`From`, which I _think_ is the case now. Let me know if there's
something I'm not thinking of and I can try and make it smarter.
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