Add FRAC_1_SQRT_2PI constant to f16/f32/f64/f128

[sunsided]

This adds the FRAC_1_SQRT_2PI to the f16, f32, f64 and f128 as 1/√(2π). The rationale is that while FRAC_1_SQRT_PI already exists, Gaussian calculations for random normal distributions require a 1/(σ√(2π)) term, which could then be directly expressed e.g. as f32::FRAC_1_SQRT_2PI / sigma.

The actual value is approximately 1/√(2π) = 0.3989422804014326779399460599343818684758586311649346576659258296…. Truncated/rounded forms were used for the individual types.

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I did not any of the #[unstable] attributes since I am not aware of their implications.

Edit: I applied the stability attributes from the surrounding types according to what seemed most likely correct. I believe the more_float_constants feature marker is incorrectly applied, but I wasn't sure how to proceed.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) some time within the next two weeks.

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[Mark-Simulacrum]

New unstable API surface area currently requires an ACP. Please file one and if it's approved we can merge.

@rustbot author

[sunsided]

Thanks, added it at rust-lang/libs-team#383. Exciting!

@rustbot review

[sunsided]

I added #[doc(alias = "FRAC_1_SQRT_TAU")] to the constants to create a bit of symmetry with the already-existing TAU constant, as discussed in rust-lang/libs-team#383.