Unrolled build for rust-lang#141282

Rollup merge of rust-lang#141282 - DJMcNab:core-float-math-math, r=tgross35

`core_float_math`: Move functions to `math` module

When these functions were added in rust-lang#138087 It made a relatively common pattern for emulating these functions using an extension trait (which internally uses `libm`) much more fragile. If `core::f32` happened to be imported by the user (to access a constant, say), then that import in the module namespace would take precedence over the `f32` in the type namespace for resolving these functions, running headfirst into the stability attribute.

We ran into this in [Color](https://github.com/linebender/color) and chose to release the remedial 0.3.1 and 0.2.4, to allow downstream crates to build on `docs.rs`.

As these methods are perma-unstable, moving them into a new module should not have any long-term concerns, and ensures that this "breakage" doesn't adversely impact anyone else.

I believe that I've made the module unstable correctly. I presume that this does not require a test to make sure stable code can't depend on the module existing?

I've left the stability attribute on each function - happy to tweak this if a different pattern is more correct.

Tracking issue for `core_float_math`: rust-lang#137578.
This PR is as requested in rust-lang#138087.

r? `@tgross35`

Recommended reviewing with whitespace hidden.

(This is my first PR to `std/core`/this repository, as far as I can remember)

Author: rust-timer