Begin implementation of ndb.Model (googleapis#6581)

Also 

- Fixing `repr()` of `Text/StringProperty`
- Adding coverage for `MetaModel.__repr__`

For now, reducing the functionality for `Model._set_projection`.
This is because a `StructuredProperty` is needed to really be able
to test / run `_set_projection` as written.

- Add a virtual `Model._put` so we can refer to it in docs
- Add a custom docstring for `Model.key`, since before Sphinx was
  trying to render the `ModelKey` docstring in the context of `Model`,
  which caused a failure of `.. automethod:: _validate`

MIGRATION_NOTES.md

@@ -119,6 +119,9 @@ The primary differences come from:
   has new support for adding such a user to a `google.cloud.datastore.Entity`
   and for reading one from a new-style `Entity`
 - The `UserProperty` class no longer supports `auto_current_user(_add)`
+- `Model.__repr__` will use `_key` to describe the entity's key when there
+  is also a user-defined property named `key`. For an example, see the
+  class docstring for `Model`.
 
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