traits: add `to_degrees` and `to_radians` on `Float`
traits: add `to_degrees` and `to_radians` on `Float`

To avoid a breaking change, these have crude default implementations as
well as better implementations for `f32` and `f64` in particular.  They
don't use the inherent methods though, because `f32` didn't stabilize
those until Rust 1.7.

Fixes #211

rust-num/num-traits,rust-num/num,rust-num/num-traits,rust-num/num-integer,rust-lang/num,koverstreet/num,rust-num/num,DonSheddow/num,rust-num/num-bigint,vks/num,rust-num/num-integer,rust-lang/num,rust-num/num-bigint,vks/num,DonSheddow/num,koverstreet/num