Fixing Curve trait not being object safe. (#14939)

# Objective

- `Curve<T>` was meant to be object safe, but one of the latest commits
made it not object safe.
- When trying to use `Curve<T>` as `&dyn Curve<T>` this compile error is
raised:
```
error[E0038]: the trait `curve::Curve` cannot be made into an object
    --> crates/bevy_math/src/curve/mod.rs:1025:20
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
    --> crates/bevy_math/src/curve/mod.rs:60:8
     |
23   | pub trait Curve<T> {
     |           ----- this trait cannot be made into an object...
...
60   |     fn sample_iter(&self, iter: impl IntoIterator<Item = f32>) -> impl Iterator<Item = Option<T>> {
     |        ^^^^^^^^^^^                                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ...because method `sample_iter` references an `impl Trait` type in its return type
     |        |
     |        ...because method `sample_iter` has generic type parameters
...
```

## Solution

- Making `Curve<T>` object safe again by adding `Self: Sized` to newly
added methods.

## Testing

- Added new test that ensures the `Curve<T>` trait can be made into an
objet.

crates/bevy_math/src/curve/mod.rs

@@ -57,7 +57,10 @@ pub trait Curve<T> {
     /// The samples are returned in the same order as the parameter values `t_n` were provided and
     /// will include all results. This leaves the responsibility for things like filtering and
     /// sorting to the user for maximum flexibility.
-    fn sample_iter(&self, iter: impl IntoIterator<Item = f32>) -> impl Iterator<Item = Option<T>> {
+    fn sample_iter(&self, iter: impl IntoIterator<Item = f32>) -> impl Iterator<Item = Option<T>>
+    where
+        Self: Sized,
+    {
         iter.into_iter().map(|t| self.sample(t))
     }
 
@@ -72,10 +75,10 @@ pub trait Curve<T> {
     /// The samples are returned in the same order as the parameter values `t_n` were provided and
     /// will include all results. This leaves the responsibility for things like filtering and
     /// sorting to the user for maximum flexibility.
-    fn sample_iter_unchecked(
-        &self,
-        iter: impl IntoIterator<Item = f32>,
-    ) -> impl Iterator<Item = T> {
+    fn sample_iter_unchecked(&self, iter: impl IntoIterator<Item = f32>) -> impl Iterator<Item = T>
+    where
+        Self: Sized,
+    {
         iter.into_iter().map(|t| self.sample_unchecked(t))
     }
 
@@ -85,7 +88,10 @@ pub trait Curve<T> {
     /// The samples are returned in the same order as the parameter values `t_n` were provided and
     /// will include all results. This leaves the responsibility for things like filtering and
     /// sorting to the user for maximum flexibility.
-    fn sample_iter_clamped(&self, iter: impl IntoIterator<Item = f32>) -> impl Iterator<Item = T> {
+    fn sample_iter_clamped(&self, iter: impl IntoIterator<Item = f32>) -> impl Iterator<Item = T>
+    where
+        Self: Sized,
+    {
         iter.into_iter().map(|t| self.sample_clamped(t))
     }
 
@@ -760,6 +766,9 @@ where
 /// must be left-finite.
 ///
 /// Curves of this type are produced by [`Curve::chain`].
+#[derive(Clone, Debug)]
+#[cfg_attr(feature = "serialize", derive(serde::Serialize, serde::Deserialize))]
+#[cfg_attr(feature = "bevy_reflect", derive(Reflect))]
 pub struct ChainCurve<T, C, D> {
     first: C,
     second: D,
@@ -1013,6 +1022,15 @@ mod tests {
     use approx::{assert_abs_diff_eq, AbsDiffEq};
     use std::f32::consts::TAU;
 
+    #[test]
+    fn curve_can_be_made_into_an_object() {
+        let curve = constant_curve(Interval::UNIT, 42.0);
+        let curve: &dyn Curve<f64> = &curve;
+
+        assert_eq!(curve.sample(1.0), Some(42.0));
+        assert_eq!(curve.sample(2.0), None);
+    }
+
     #[test]
     fn constant_curves() {
         let curve = constant_curve(Interval::EVERYWHERE, 5.0);