- #7091
- add `IKeyRewriterValueProvider` to remove rewritten keys or value providers containing such keys
- similar to `IBindingSourceValueProvider` except `CompositeValueProvider` keeps non-implementers around
- remove `after.Order == before.Order` special cases
- a premature optimization that could lead to lost inner provider replacements
- rework `EnumerableValueProviderTest` to ease test override in `CompositeValueProviderTest`
- add `EmptyValueProvider` fields to reduce `CompositeValueProvider` allocations
nits:
- remove Linq use in `CompositeValueProvider`
- do not create an unnecessary dictionary in `CompositeValueProvider.Filter(...)` methods
- accept VS suggestions, mostly pattern matching
