Auto merge of #29530 - jseyfried:resolve, r=nikomatsakis

Replace `TypeNsDef` and `ValueNsDef` with a more general type `NsDef`.

Define a newtype `NameBinding` for `Rc<RefCell<Option<NsDef>>>` and refactor `NameBindings` to be a `NameBinding` for each namespace.

Replace uses of `NameBindings` with `NameBinding` where only one binding is being used (in `NamespaceResult`, `Target,` etc).

Refactor away `resolve_definition_of_name_in_module` and `NameDefinition`, fixing issue #4952.

Author: bors