Merge #23

23: Add RealNum trait for real data types (Float, but without floating-point specific features) r=cuviper a=yoanlcq

This is supposed to fix [#19](#19); I assumed going ahead would be better than bumping the thread.  

In any case, I understand that it is a quite significant addition and won't mind too much if it doesn't make it.

This adds a new `RealNum` trait, along with a universal impl `impl<T: Float> RealNum for T { ... }`.  
Therefore, this shouldn't be a breaking change, except in places where both traits are imported (which obviously only happened in a few places in this crate).

The intent is that generic code may prefer to use `RealNum` instead of `Float` when floating-point isn't a requirement. In the future (next major version ?), I guess `Float` could be made to only provide floating-point-specific features on top of `RealNum`.

Most of the code+doc was copy-pasted from `Float`, but the doc comments should be up-to-date with the situation; `Float` only makes an appearance when talking about NaN and infinity.

Issues I've seen : 
- `RealNum` might not be the name we want;
- I've mentioned that `sqrt()` is allowed to panic if the input is negative and has no meaningful NaN representation;
- Should we do that too for e.g `log()` ? Like `sqrt()`, it's supposed to return Nan when `x < 0`.

Thanks for your time. :)

Author: bors[bot]

src/lib.rs

@@ -19,6 +19,7 @@ use std::fmt;
 
 pub use bounds::Bounded;
 pub use float::{Float, FloatConst};
+// pub use real::Real; // NOTE: Don't do this, it breaks `use num_traits::*;`.
 pub use identities::{Zero, One, zero, one};
 pub use ops::checked::*;
 pub use ops::wrapping::*;
@@ -33,6 +34,7 @@ pub mod sign;
 pub mod ops;
 pub mod bounds;
 pub mod float;
+pub mod real;
 pub mod cast;
 pub mod int;
 pub mod pow;