Stabilise `EULER_GAMMA` and `GOLDEN_RATIO` constants for `f32` and `f64`.
Tracking issue: rust-lang/rust#146939
This PR renames the `EGAMMA` and `PHI` constants to `EULER_GAMMA` and `GOLDEN_RATIO` – respectively – and stabilises them under these names for `f32` and `f64`:
```rust
// core::f16::consts
pub const GOLDEN_RATIO: f16;
pub const EULER_GAMMA: f16;
// core::f32::consts
pub const GOLDEN_RATIO: f32;
pub const EULER_GAMMA: f32;
// core::f64::consts
pub const GOLDEN_RATIO: f64;
pub const EULER_GAMMA: f64;
// core::f128::consts
pub const GOLDEN_RATIO: f128;
pub const EULER_GAMMA: f128;
```
The feature gate for the stabilised items is also changed to `euler_gamma_golden_ratio`.
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