Don't depend on `std` to sort floats

[veddan]

The Float trait used by sort_floats has been move to then num crate, which depends on std. I don't want this, and don't use any functionality related to std.

I'm not sure what the best solution is. Writing my own Float will solve the immediate issue (being able to sort &[f64]/&[f32], but will hurt interoperability in generic code.

[lambda-fairy]

rust-num/num#216 might be relevant.

[vks]

Float cannot be ported to core without reimplementing almost all math functions! Only a few are implemented in core.