…ine#15493)

(Note: bevyengine#15434 implements something very similar to this for functional
curve adaptors, which is why they aren't present in this PR.)

# Objective

Previously, there was basically no chance that the
explicitly-interpolating sample curve structs from the `Curve` API would
actually be `Reflect`. The reason for this is functional programming:
the structs contain an explicit interpolation `I: Fn(&T, &T, f32) -> T`
which, under typical circumstances, will never be `Reflect`, which
prevents the derive from realistically succeeding. In fact, they won't
be a lot of other things either, notably including both`Debug` and
`TypePath`, which are also required for reflection to succeed.

The goal of this PR is to weaken the implementations of reflection
traits for these structs so that they can implement `Reflect` under
reasonable circumstances. (Notably, they will still not be
`FromReflect`, which is unavoidable.)

## Solution

The function fields are marked as `#[reflect(ignore)]`, and the derive
macro for `Reflect` has `FromReflect` disabled. (This is not fully
optimal, but we don't presently have any kind of "read-only" attribute
for these fields.) Additionally, these structs receive custom `Debug`
and `TypePath` implementations that display the function's (unstable!)
type name instead of its value or type path (respectively). In the case
of `TypePath`, this is a bit janky, but the instability of `type_name`
won't generally present an issue for generics, which would have to be
registered manually in the type registry anyway, which is impossible
because the function type parameters cannot be named.

(And in general, the "blessed" route for such cases would generally
involve manually monomorphizing the function parameter away, which also
allows access to `FromReflect` etc. through very ordinary use of the
derive macro.)

## Testing

Tests in the new `bevy_math::curve::sample_curves` module guarantee that
these are actually `Reflect` under reasonable circumstances.

---

## Future changes

If and when function item types become `Default`, these types will need
to receive custom `FromReflect` implementations that exploit it. Such a
custom implementation would also be desirable if users start doing
things like wrapping function items in `Default`/`FromReflect` wrappers
that still implement a `Fn` trait.

Additionally, if function types become nameable in user-space, the
stance on `Debug`/`TypePath` may bear reexamination, since partial
monomorphization through wrappers would make implementing reflect traits
for function types potentially more viable.

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Co-authored-by: Gino Valente <[email protected]>