Rollup merge of #161250 - okaneco:nan_to, r=clarfonthey

core/num: Implement feature `float_nan_to`

Accepted ACP: https://github.com/rust-lang/libs-team/issues/787
Tracking issue: https://github.com/rust-lang/rust/issues/161248

Add function `nan_to` on `f16`, `f32`, `f64`, `f128` which replaces NaN values with a user-specified value or returns the original value if it is not a NaN.

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No LLM use.