auto merge of #6263 : bjz/rust/approx-eq, r=erickt

Moving the trait into `core` allows it to be added to the `num::Float` trait.

I have also added and `assert_approx_eq!` macro. This is useful fo making approximate assertions on types that implement the `ApproxEq` trait.

Examples:

~~~rust
// using the default epsilon value
assert_approx_eq!(1.0000001f, 1.0f);

// using a custom epsilon value
assert_approx_eq!(1.000001f, 1.0f, 1.0e-5);

// fails with: "left: 1.00001 does not approximately equal right: 1"
assert_approx_eq!(1.00001f, 1.0f);
~~~

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