Implement to_radians and to_degrees for Float

[frewsxcv]

https://doc.rust-lang.org/nightly/std/primitive.f64.html#method.to_degrees

https://doc.rust-lang.org/nightly/std/primitive.f64.html#method.to_radians

To better mirror what's offered in the standard library, it'd be nice if these methods were available for any Float.

[cuviper]

Curious that those are missing. Is there anything else in f32 and f64 that we lack?

It's a breaking change to add methods to a trait, unless you provide a default implementation. Is there a way to generically write these? Then f32 and f64 can override with their native versions, of course.

[frewsxcv]

Relevant discussion in georust/geo#48 by the way. Feel free to take any of my code I wrote.

[cuviper]

Ugh. Rust 1.0 had these stable for f64, but not until 1.7 for f32 . (rust-lang/rust#30672)

[frewsxcv]

What would be the next step forward for this? In what way would it be a breaking change? Do you think it'd be possible to write a default implementation?

[cuviper]

It's a little annoying since their Float implementations are shared, but I can code it openly for now. And I think I've got a way to write a default too, so it's not a breaking change.

[cuviper]

Please let me know if #212 looks ok to you.