Disallow `fn_to_numeric_cast_any` once merged upstream

[michaelsproul]

Description

I've authored a Clippy lint to detect a case of bugs that we should enable once it's available in rustc stable: rust-lang/rust-clippy#7705

Optimistically it might be available in Rust 1.57.0 on Dec 2nd/3rd

[michaelsproul]

I think the PR got merged in time for Rust 1.57.0. It's in Nightly at the moment: https://rust-lang.github.io/rust-clippy/master/index.html#fn_to_numeric_cast_any

[michaelsproul]

Ah, looks like it might be 1.58.0 (January)