Replace mesh IndicesIter with Either iterator

[stepancheg]

Objective

• Less code
• Also I wanted to use either iterator in another place in bevy_mesh (for changes like Add a test for Mesh::triangles and fix for TriangleStrip #16026)

Solution

• Replace IndicesIter with either
• Bevy already depends on either crate:

bevy/crates/bevy_asset/Cargo.toml

Line 40 in ec26842

either = "1.13"

Testing

CI

[alice-i-cecile]

No strong feelings here 🤔 It's nice to reduce this, but the dependency isn't 0 cost, even if it is used elsewhere in Bevy already.

[Victoronz]

This change makes me wonder one thing:
Returning impl Iterator seems wasteful here, since all of the trait impls that the iterator type has are lost, yet the current actual type name isn't actually more complex to write directly.
If we replace the IndicesIter enum with either, we suddenly lose even more trait impls, when we could be returning a type alias instead (though this would regress error messages somewhat, since those expand type aliases).
F.e. this would allow calling len() (ExactSizeIterator), or rev() (DoubleEndedIterator) on the return type, and a lot more.
As Alice said, either isn't free, and we're not even exposing the benefits of using it here to the API consumer.

[stepancheg]

@Victoronz it was impl Iterator before this change.

If we need more precise trait, we can return impl ExactSizeIterator + FusedIterator. But practically it does not matter.

We can even do + DoubleEndedIterator if we need to, which we get for free with either, but don't have in current impl.

Anyway, if this PR is not a big deal. Definitely not worth a long discussion.