schemagen: fix nullable root types (#13)

The C++ code generator now handles schemas where the top-level type is
nullable ("type": ["object", "null"], ["string", "null"], or
["array", "null"]).

Changes to weaseljson_schemagen.py:
- Rename the inner object struct when the root is a nullable object, so
  the `using Root = std::optional<...>` alias no longer conflicts with
  `struct Root`.
- Treat nullable root objects and arrays as container roots, emplacing
  the inner value before pushing the root frame and pointing the frame at
  the contained value.
- For nullable root scalars/enums, engage() now returns a pointer to the
  value inside the optional rather than to the optional wrapper itself.
- cbNull() now safely accepts a top-level null when the root is nullable
  and rejects it otherwise.

Regression tests added:
- nullable_object.schema.json + test_nullable_root.cpp
- nullable_string.schema.json
- nullable_array.schema.json

Closes #13

contrib/schemagen/nullable_object.schema.json

@@ -0,0 +1,15 @@
+{
+  "type": [
+    "object",
+    "null"
+  ],
+  "additionalProperties": false,
+  "required": [
+    "x"
+  ],
+  "properties": {
+    "x": {
+      "type": "string"
+    }
+  }
+}