auto merge of #6110 : bjz/rust/numeric-traits, r=pcwalton

As discussed on issue #4819, I have created four new traits: `Algebraic`, `Trigonometric`, `Exponential` and `Hyperbolic`, and moved the appropriate methods into them from `Real`.

~~~rust
pub trait Algebraic {
    fn pow(&self, n: Self) -> Self;
    fn sqrt(&self) -> Self;
    fn rsqrt(&self) -> Self;
    fn cbrt(&self) -> Self;
    fn hypot(&self, other: Self) -> Self;
}

pub trait Trigonometric {
    fn sin(&self) -> Self;
    fn cos(&self) -> Self;
    fn tan(&self) -> Self;
    fn asin(&self) -> Self;
    fn acos(&self) -> Self;
    fn atan(&self) -> Self;
    fn atan2(&self, other: Self) -> Self;
}

pub trait Exponential {
    fn exp(&self) -> Self;
    fn exp2(&self) -> Self;
    fn expm1(&self) -> Self;
    fn log(&self) -> Self;
    fn log2(&self) -> Self;
    fn log10(&self) -> Self;
}

pub trait Hyperbolic: Exponential {
    fn sinh(&self) -> Self;
    fn cosh(&self) -> Self;
    fn tanh(&self) -> Self;
}
~~~

There was some discussion over whether we should shorten the names, for example `Trig` and `Exp`. No abbreviations have been agreed on yet, but this could be considered in the future.

Additionally, `Integer::divisible_by` has been renamed to `Integer::is_multiple_of`.

src/libcore/prelude.rs

@@ -39,8 +39,9 @@ pub use old_iter::{CopyableIter, CopyableOrderedIter, CopyableNonstrictIter};
 pub use old_iter::{ExtendedMutableIter};
 pub use iter::Times;
 pub use num::{Num, NumCast};
-pub use num::{Orderable, Signed, Unsigned, Integer};
-pub use num::{Round, Fractional, Real, RealExt};
+pub use num::{Orderable, Signed, Unsigned, Round};
+pub use num::{Algebraic, Trigonometric, Exponential, Hyperbolic};
+pub use num::{Integer, Fractional, Real, RealExt};
 pub use num::{Bitwise, BitCount, Bounded};
 pub use num::{Primitive, Int, Float};
 pub use path::GenericPath;