- part I of II for #2445 (with a duplicate code PR to follow)
- needed for #2445 because new `ModelState` entries for values will make inconsisteny worse
- change `BodyModelBinder` to use same keys for all `ModelBindingResult`s and `ModelState` entries
- return fatal error result if formatter adds an error to `ModelState`
- update potential callers to avoid avoid ignoring `IsFatalError`
- fix test attempting to serialize all of `ModelState`
- will be borked with additional `RawValue`s in state
- fix two other tests that serialized `ModelState` but checked only `IsValid`
nits:
- address minor inconsistencies in `ModelBindingContext`
- use `System.Reflection.Extensions` package a bit more, where it's already referenced
- remove some unused resources
