Add associated consts to f32, f64 for mathematical constants

Proposal

Problem statement

Currently, to use mathematical constants such as π, you need to use std::f32::consts::PI.
This proposal is for adding associated constants to the f32 and f64 types so that the constants could be accessed with just f32::PI for example.

Motivation, use-cases

1. std::f32::consts::PI is quite long. It takes a while to type out, and makes code difficult to read if written out in full everywhere.
   Consider

   assert_eq!(std::f32::consts::FRAC_PI_4.sin(), std::f32::consts::FRAC_1_SQRT_2);
   

   vs

   assert_eq!(f32::FRAC_PI_4.sin(), f32::FRAC_1_SQRT_2);
   

   You can always do something like use std::f32::consts as f32c; but it would be nice for a shorter path to be included in the standard library.
2. I think this is where first-time users of Rust would expect mathematical constants to be. I know that when I first wanted to use π in a rust program, I wrote out f32::PI and to my disappointment it didn't work and I had to do an internet search to find out the correct path.
3. NAN, INFINITY, etc. are associated constants, and it feels inconsistent to me for f32::NAN to exist, but not f32::PI.

Solution sketches

I have implemented this on my fork: https://github.com/rust-lang/rust/compare/master...pommicket:rust:shorter-paths-for-float-consts

Links and related work

• The popular crates half and fixed already have these as associated constants on their types.
• Rust internals thread: https://internals.rust-lang.org/t/should-pi-be-available-as-f32-pi-etc/18670
• [Edit:] RFC pull request: https://github.com/rust-lang/rfcs/pull/3418

What happens now?

This issue is part of the libs-api team API change proposal process. Once this issue is filed the libs-api team will review open proposals in its weekly meeting. You should receive feedback within a week or two.

Start with the API change proposal process and the RFC pull request, then compare the existing std::f32::consts paths with the fork implementation linked in the proposal. Done means the libs-api team has reviewed the proposal and agreed on whether f32 and f64 associated mathematical constants should be added.