different behavior of `to_f32` and `to_f64`

[Shoeboxam]

I just wanted to check if it is intentional to have to_f32 respect the rounding mode of self, but to_f64 always round to HalfEven. I think the API would be less surprising if these behaved the same way, and more useful if to_f64 were adjusted to use the same rounding mode as self.

Thanks as always!