Change FloatDiff to take its arguments by value

[jtempest]

The FloatDiff trait as of the 0.2 release is specified in a non-idiomatic fashion when compared to Rust's standard arithmetic operations, in particular std::ops::Sub, which it is analogous to. This makes it difficult to implement over iterable types. Additionally, since it takes other by reference, it is impossible to implement the following common pattern (see also forward_ref_binop), which can be important for certain optimisations:

impl Sub for MyType
impl Sub for &MyType
impl Sub<&MyTrait> for MyType
impl Sub<&MyTrait> for &MyType

The other consideration is whether FloatDiff ought to be two different traits, one per kind of difference op. Since its primary purpose in this library is to provide debug context information to assert_float_eq and friends, for the moment I'm content to leave it as a single trait, since it compels an implementor to provide all of the required information.

Therefore, the work to be done is that FloatDiff should be changed to the following:

pub trait FloatDiff<Rhs = Self> {
    type AbsDiff;
    type UlpsDiff;

    fn abs_diff(self, other: Rhs) -> Self::AbsDiff;
    fn ulps_diff(self, other: Rhs) -> Self::UlpsDiff;
}

This would necessitate changing the existing implementations (f32, f64, Complex and [T; n]) to take this new form. As part of this change, I'd also like to implement the trait across LHS and RHS references for those types (as Sub is over primitive types).

[jtempest]

The assert macros which use FloatDiff require it to use reference versions without significantly more documentation/expectation on the user. That means that most often FloatDiff<&A> &B would be the implemented version to enable assertions, which isn't significantly different to the current formulation, but provides more potential pitfalls/onus on the implementor. Since FloatDiff is entirely used by the library for debug info, I'm going to leave it as-is. If the library was more general, it would be a more pressing change.