Mutable NestedQuery with wrapper that erases QueryData type

[chescock]

Objective

Support mutable queries using NestedQuery.

The reason this was not supported in the initial version is that we want NestedQuery<D, F>::ReadOnly == NestedQuery<D::ReadOnly, F>, but that doesn't satisfy the ReadOnly::State == State constraint since QueryState<D::ReadOnly, F> != QueryState<D, F>.

Solution

Store QueryState<D::ReadOnly, F> in the State. Since D::ReadOnly::ReadOnly == D::ReadOnly, that ensures the states match.

In order to simplify the safety proofs and ensure we always convert it back to the appropriate type, introduce an ErasedQueryState wrapper type that prevents accidental access to the inner QueryState<D::ReadOnly, F>.

Adopted and modified from #25642. See also #25652 for context.

[hymm]

code looks good, but can we add some tests for the new functionality

[chescock]

code looks good, but can we add some tests for the new functionality

What sort of tests do you have in mind? Note that I did extend the doc tests! There's an example of Query<NestedQuery<&mut A>> that shows that iter() and get_mut() work but iter_mut() doesn't, and the Parent<D, F> example now uses QueryData instead of ReadOnlyQueryData.

... Oh, the actual usage for Parent is still &Data, though. Let me use &mut Data there, too, and add some docs for how to set type ReadOnly.