Allow UnevenSampleCurve to infer interpolation arguments from samples

[laundmo]

Objective

bevy_math::curve::UnevenSampleCurve was lacking trait bounds on its new method which caused a need to manually specify interpolation function argument types, even though the types are already known from the timed_samples argument.

UnevenSampleCurve::new(
    [(0.1, 1.0), (1.0, 3.0)],
    |x: &f64, y: &f64, t| x.lerp(*y, t)
//      ^ ugly and annoying
)

Solution

This adds the Fn trait bound to the new method, matching the behaviour of SampleCurve.

Testing

I added a test which doesn't actually test for any runtime behaviour, instead writing code that would error before this PR.

---

Showcase

It's now possible to create a UnevenSampleCurve without specifying the interpolation closure argument types.

UnevenSampleCurve::new(
    [(0.1, 1.0), (1.0, 3.0)],
-   |x: &f64, y: &f64, t| x.lerp(*y, t)
+   |x, y, t| x.lerp(*y, t)
)

Sidenote

Seeing x and y in lerp feels wrong, I've always used a and b to avoid confusion with coordinates.

[mweatherley]

Nice!