add missing Iterator::size_hint overrides, most of which cause ExactSizeIterator::len to panic

[JonasJebing]

Objective

• Fixes ExactSizeIterator::len panic in a few of these iterators. Note: the std default impl of ExactSizeIterator::len has assert_eq!(Some(lower), upper);. I am facing this issue myself with <index_set::Iter as ExactSizeIterator>::len.
• Should slightly improve performance in some cases, where the more precise size_hint causes less allocations (for example when collecting into a Vec)

Solution

• Add size_hint overrides that forward to inner iterators or an ExactSizeIterator::len override in one specific case.

Testing

• Since the overrides are pretty simple, I mostly didn't add any tests.
• I didn't feel 100% confident in the WriteBatchIds::size_hint implementation, so I added a test there.

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[Victoronz]

Great catch! I was trying to figure out where the performance mismatch I was seeing between these wrappers and their inner types came from, and missing overrides like this, at least in part, look to be a likely answer! (TrustedLen and inlining behavior are some of my other suspects)

Seems like other entity module iterators like UniqueEntityIter and UniqueEntitySliceIter didn't miss out on this specific override, but a general pass should be made through the entire module to properly pipe through trait method overrides.

[JonasJebing]

@Victoronz I did do a search for Iterator for in the entire bevy code base and fixed all instances that I saw that way. I'm reasonably confident, that I found all instances out of those search results where the Iterator::next implementation is not more complex than self.0.next().

Edit for clarity: I didn't check whether any other overrides than size_hint would be needed anywhere.