Fix incorrect replacement of `modulo` with `rem`
Move impls of `Num` out of core::num and clean up imports
Use #[cfg(not(stage0))] to exclude items from stage0

As requested on the mailing list: https://mail.mozilla.org/pipermail/rust-dev/2013-April/003713.html
Rename Natural to Integer

'Natural' normally means 'positive integer' in mathematics. It is therefore strange to implement it on signed integer types. 'Integer' is probably a better choice.
Use borrowed pointers for Integer methods

This brings them in line with the quot and rem traits, and is be better for large Integer types like BigInt and BigUint because they don't need to be copied unnecessarily.
Add Fractional, Real and RealExt traits
Restore Round trait and move appropriate methods out of Real
Update impl of Round for Ratio
auto merge of #6048 : bjz/rust/numeric-traits, r=pcwalton

As part of the numeric trait reform (see issue #4819), I have added the following traits to `core::num` and implemented them for floating point types:

~~~rust
pub trait Round {
    fn floor(&self) -> Self;
    fn ceil(&self) -> Self;
    fn round(&self) -> Self;
    fn trunc(&self) -> Self;
    fn fract(&self) -> Self;
}

pub trait Fractional: Num
                    + Ord
                    + Round
                    + Quot<Self,Self> {
    fn recip(&self) -> Self;
}

pub trait Real: Signed
              + Fractional {
    // Common Constants
    fn pi() -> Self;
    fn two_pi() -> Self;
    fn frac_pi_2() -> Self;
    fn frac_pi_3() -> Self;
    fn frac_pi_4() -> Self;
    fn frac_pi_6() -> Self;
    fn frac_pi_8() -> Self;
    fn frac_1_pi() -> Self;
    fn frac_2_pi() -> Self;
    fn frac_2_sqrtpi() -> Self;
    fn sqrt2() -> Self;
    fn frac_1_sqrt2() -> Self;
    fn e() -> Self;
    fn log2_e() -> Self;
    fn log10_e() -> Self;
    fn log_2() -> Self;
    fn log_10() -> Self;

    // Exponential functions
    fn pow(&self, n: Self) -> Self;
    fn exp(&self) -> Self;
    fn exp2(&self) -> Self;
    fn expm1(&self) -> Self;
    fn ldexp(&self, n: int) -> Self;
    fn log(&self) -> Self;
    fn log2(&self) -> Self;
    fn log10(&self) -> Self;
    fn log_radix(&self) -> Self;
    fn ilog_radix(&self) -> int;
    fn sqrt(&self) -> Self;
    fn rsqrt(&self) -> Self;
    fn cbrt(&self) -> Self;

    // Angular conversions
    fn to_degrees(&self) -> Self;
    fn to_radians(&self) -> Self;

    // Triganomic functions
    fn hypot(&self, other: Self) -> Self;
    fn sin(&self) -> Self;
    fn cos(&self) -> Self;
    fn tan(&self) -> Self;

    // Inverse triganomic functions
    fn asin(&self) -> Self;
    fn acos(&self) -> Self;
    fn atan(&self) -> Self;
    fn atan2(&self, other: Self) -> Self;

    // Hyperbolic triganomic functions
    fn sinh(&self) -> Self;
    fn cosh(&self) -> Self;
    fn tanh(&self) -> Self;
}

/// Methods that are harder to implement and not commonly used.
pub trait RealExt: Real {
    // Gamma functions
    fn lgamma(&self) -> (int, Self);
    fn tgamma(&self) -> Self;

    // Bessel functions
    fn j0(&self) -> Self;
    fn j1(&self) -> Self;
    fn jn(&self, n: int) -> Self;
    fn y0(&self) -> Self;
    fn y1(&self) -> Self;
    fn yn(&self, n: int) -> Self;
} 
~~~

The constants in `Real` could be [associated items](http://smallcultfollowing.com/babysteps/blog/2013/04/03/associated-items-continued/) in the future (see issue #5527). At the moment I have left the constants in `{float|f32|f64}::consts` in case folks need to access these at compile time. There are also instances of `int` in `Real` and `RealExt`. In the future these could be replaced with an associated `INTEGER` type on `Real`.

`Natural` has also been renamed to `Integer`. This is because `Natural` normally means 'positive integer' in mathematics. It is therefore strange to implement it on signed integer types. `Integer` is probably a better choice.

I have also switched some of the `Integer` methods to take borrowed pointers as arguments. This brings them in line with the `Quot` and `Rem` traits, and is be better for large Integer types like `BigInt` and `BigUint` because they don't need to be copied unnecessarily.

There has also been considerable discussion on the mailing list and IRC about the renaming of the `Div` and `Modulo` traits to `Quot` and `Rem`. Depending on the outcome of these discussions they might be renamed again.