Integrate common floating point constants

[aomader]

In most of my implementations involving generic floating point values I have the need to work with pi and/or e. Thus it would be really helpful to provide static functions to retrieve those values for the concrete type.

[cuviper]

In general, let e = Float::one().exp(); and let pi = (-Float::one()).acos(); should work. We could provide these as default implementations and override the specific constants for f32 and f64.

[cuviper]

Associated constants would be nice too, rust-lang/rust#29646, but we couldn't provide defaults there.

[IvanUkhov]

I agree that pretty much every mathematical constant can be computed using the methods that are already there. However, (-Float::one()).acos() and alike obscure the actual algorithm. Do we really want to see people in the Rust community writing this kind of generic code? Besides, who is willing to guarantee that (-Float::one()).acos() is indeed a zero-cost abstraction?

[cuviper]

No, I don't expect people should do that -- it's just a way to write a generic default so it's not a breaking change. We can provide the proper constants in the implementation for the concrete float types.

Care to write up a PR?

[IvanUkhov]

@cuviper, first of all, I apologize if it sounded rude; no offense was intended. I’m an active user of num and really appreciate your work.

Yes, absolutely. If there are no objections, I’ll be happy to help.

[cuviper]

No problem, sorry if I sounded too defensive. :)

[cuviper]

Fixed by #220 -- thanks @IvanUkhov!