Provide functions for checking raw floating point values for equality.

[olson-sean-k]

Hashing functions like hash_float_array are provided because sometimes it is not possible or ergonomic to use wrapper types within another type. README.md mentions this example:

use decorum;

#[derive(Derivative)]
#[derivative(Hash)]
pub struct Vertex {
    #[derivative(Hash(hash_with = "decorum::hash_float_array"))]
    pub position: [f32; 3],
    ...
}

This vertex type may be fed to graphics code that expects raw floating point data (for example, see the gfx pipeline macros and shader compilers).

A similar problem exists for implementing Eq: if it is not possible or ergonomic to use wrapper types, there is currently no convenient way to implement Eq. This can be done via conversions, but that gets messy fairly quickly. Instead, Decorum should provide eq_float, eq_float_slice, and eq_float_array functions that are analogous to the hashing functions.

[olson-sean-k]

Once this is available, the above example would look something like this:

use decorum;

#[derive(Derivative)]
#[derivative(Eq, Hash, PartialEq)]
pub struct Vertex {
    #[derivative(Hash(hash_with = "decorum::hash_float_array"))]
    #[derivative(PartialEq(compare_with = "decorum::eq_float_array"))]
    pub position: [f32; 3],
    ...
}

[olson-sean-k]

These functions are exposed by 8fdd430.