Add FloatCore::round_ties_even

[bushrat011899]

Objective

Since Rust 1.77, round_ties_even was added to f32 and f64 in the standard library. This function is used within WGPU, and as a part of a push for no_std support, this function will need to be shimmed. It could be shimmed within Naga, but this functionality may have value to other users of num-traits.

Solution

• Added round_ties_even to FloatCore with a default implementation (not a breaking change)
• Added a new feature, msrv_1_77, which allows overriding the default implementation of FloatCore::round_ties_even for f32 and f64 with the implementation from std (not a breaking change)
• Added a test ensuring the fallback implementation of round_ties_even matches the implementation in std for the f32 type (assuming it is equally valid for f64)

---

Notes

• The msrv_1_77 feature increases MSRV to 1.77, which is a typical arrangement (e.g., serde features will typically elevate MSRV). If/when num-traits advances past MSRV 1.77, this feature can become a no-op without any breaking changes.
• This is my first contribution to num-traits, please let me know if there's anything I can do to help!