Implement JsonSchema for chrono types

Requires chrono feature.

schemars/tests/expected/chrono-types.json

@@ -0,0 +1,42 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "ChronoTypes",
+  "type": "object",
+  "properties": {
+    "date_time": {
+      "type": "string",
+      "format": "date-time"
+    },
+    "naive_date": {
+      "type": "string",
+      "format": "date"
+    },
+    "naive_date_time": {
+      "type": "string",
+      "format": "partial-date-time"
+    },
+    "naive_time": {
+      "type": "string",
+      "format": "partial-date-time"
+    },
+    "weekday": {
+      "type": "string",
+      "enum": [
+        "Mon",
+        "Tue",
+        "Wed",
+        "Thu",
+        "Fri",
+        "Sat",
+        "Sun"
+      ]
+    }
+  },
+  "required": [
+    "date_time",
+    "naive_date",
+    "naive_date_time",
+    "naive_time",
+    "weekday"
+  ]
+}

schemars/tests/expected/schema-openapi3.json

@@ -88,6 +88,10 @@
           "items": {},
           "nullable": true
         },
+        "format": {
+          "type": "string",
+          "nullable": true
+        },
         "items": {
           "anyOf": [
             {

schemars/tests/expected/schema.json

@@ -24,6 +24,17 @@
         "integer"
       ]
     },
+    "Ref": {
+      "type": "object",
+      "properties": {
+        "$ref": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "$ref"
+      ]
+    },
     "Schema": {
       "anyOf": [
         {
@@ -113,6 +124,16 @@
             }
           ]
         },
+        "format": {
+          "anyOf": [
+            {
+              "type": "string"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
         "items": {
           "anyOf": [
             {
@@ -181,17 +202,6 @@
       },
       "additionalProperties": true
     },
-    "Ref": {
-      "type": "object",
-      "properties": {
-        "$ref": {
-          "type": "string"
-        }
-      },
-      "required": [
-        "$ref"
-      ]
-    },
     "SingleOrVec_For_InstanceType": {
       "anyOf": [
         {