Enable deriving JsonSchema for unit/newtype/tuple structs

schemars/tests/expected/struct-newtype.json

@@ -0,0 +1,5 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "Newtype",
+  "type": "integer"
+}

schemars/tests/expected/struct-normal.json

@@ -0,0 +1,17 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "Struct",
+  "type": "object",
+  "properties": {
+    "bar": {
+      "type": "boolean"
+    },
+    "foo": {
+      "type": "integer"
+    }
+  },
+  "required": [
+    "bar",
+    "foo"
+  ]
+}

schemars/tests/expected/struct-tuple.json

@@ -0,0 +1,15 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "Tuple",
+  "type": "array",
+  "items": [
+    {
+      "type": "integer"
+    },
+    {
+      "type": "boolean"
+    }
+  ],
+  "maxItems": 2,
+  "minItems": 2
+}

schemars/tests/expected/struct-unit.json

@@ -0,0 +1,5 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "Unit",
+  "type": "null"
+}