Round half to even for f32, f64

[clarfonthey]

I personally think that this would be nice to include in the standard library, considering how round half to even is the IEEE default and has some nice properties.

[Centril]

@clarcharr can you describe this operation more in-depth?

[pnkfelix]

I can expand on this. Just making a note here so I can easily find this bug later

[clarfonthey]

Closing since this is being implemented as part of rust-lang/rust#95317.