Expand EasingCurve documentation

[greeble-dev]

Objective

• Expand the documentation for EasingCurve.
  • I suspect this might have avoided the confusion in Public easing #17711.
• Also add a shortcut for simple cases.

Solution

• Added various examples and extra context.
• Implemented Curve<T> for EaseFunction.
  • This means EasingCurve::new(0.0, 1.0, EaseFunction::X) can be shortened to EaseFunction::X.
  • In some cases this will be a minor performance improvement.
  • Added test to confirm they're the same.
• Added some benchmarks for bonus points.

Side Notes

• I would have liked to rename EaseFunction to EaseFn for brevity, but that would be a breaking change and maybe controversial.
  • Also suspect EasingCurve should be EaseCurve, but say la vee.
• Benchmarks show that calling EaseFunction::Smoothstep is still slower than calling smoothstep directly.
  • I think this is because the compiler refuses to inline EaseFunction::eval.
  • I don't see any good solution - might need a whole different interface.

Testing

cargo test --package bevy_math

cargo doc --package bevy_math
./target/doc/bevy_math/curve/easing/struct.EasingCurve.html

cargo bench --package benches --bench math -- easing

[alice-i-cecile]

@fjkorf can I get your review here?

[alice-i-cecile]

I like the expanded usage docs. So-so on the benchmark; I think I'd prefer to have that in its own PR.