Auto merge of #100578 - Urgau:float-next-up-down, r=scottmcm

Add next_up and next_down for f32/f64 - take 2

This is a revival of rust-lang/rust#88728 which staled due to inactivity of the original author. I've address the last review comment.

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This is a pull request implementing the features described at rust-lang/rfcs#3173.

`@rustbot` label +T-libs-api -T-libs
r? `@scottmcm`
cc `@orlp`

Author: bors