Rework `ViewDataDictionary` constructors to ensure `ModelMetadata` is never `null`
- `ViewDataDictionary<TModel>` constructors now pass `typeof(TModel)` to base
  `protected` constructors
 - move type compatibility checks into base `ViewDataDictionary`
 - remove `ViewDataDictionary<TModel>.ModelMetadata` override
- don't retrieve `ModelMetadata` twice in a single constructor invocation
- remove newly-unused `protected` properties and use `private` fields in copy
  constructors

Address longstanding problems found
- avoid reusing `ModelMetadata` after `Model` value changes
 - `Model` and `ModelMetadata.Model` could previously get out of sync
- carry `ModelMetadata` forward from an outer scope only if `Model` matches
 - previously two scopes could have different `Model` values but share their
   `ModelMetadata` (and `ModelMetadata.Model`)
 - related to previous item but didn't require `Model` setting; switching
   to a property of the same type as containing `Model` was enough
- avoid NRE if `ViewDataDictionary<int>.Model` is read before it's written
 - problem affected all non-`Nullable` value types
- `ViewDataDictionary.ModelMetadata` setter should throw if value is `null`

nits:
- add and reword doc and code comments
 - `ViewDataDictionary<TModel>` constructors should only inherit base's
   parameter descriptions; have more information in the derived class
- make a few `ViewDataDictionary` properties get-only
- clean up `using` statements in `ViewDataDictionary<TModel>`
- make two constructors `internal`
Correct some `ViewDataDictionary` constructor calls
- ensure correct `ViewData.ModelState` value in a `RazorPage` instance
- let view components inherit `ViewData.Model` from surrounding context
- do not create an isolated `ViewDataDictionary<TModel>` in one HTML helper
 - allows previous `ViewDataDictionary<TModel>` constructor to be `internal`
  - also add comments for that constructor
- change two `ViewDataDictionary` copy constructor calls to avoid uselessly
  copying `Model` and related information from source
Correct code checking for `ViewDataDictionary.ModelMetadata==null`
- `null` checks can generally be removed but in one case needed to
  special-case metadata for `object`

nit: remove duplicate empty `string` check in
`ExpressionMetadataProvider.FromStringExpression()`
Adjust tests to handle new `ViewDataDictionary` invariants
- ensure `ViewDataDictionary` constructors are not passed a `null` or
  `Mock.Of<IModelMetadataProvider>()` instance
 - `ViewDataDictionary` constructors always use the `IModelMetadataProvider`
- `viewData.ModelMetadata` now never `null`
- `ViewDataDictionary<int>.Model` no longer throws if read before it's written
- `ViewDataDictionary.ModelMetadata` now copied to new instances in fewer cases
 - e.g. don't use unusual `object` datatype with customized `ModelMetadata`