Remove `num` crate dependency

[mitchmindtree]

The num crate is only used for a handful of cases in conrod, primarily within the utils.rs module.

It would be nice if we could drop this dependency in favour of writing our own more minimal, use-case-specific traits.

Perhaps if necessary, we could switch to depend on the num-traits crate in the meantime.

[bvssvni]

This is used by some Piston libraries https://github.com/pistondevelopers/float