Support int, string and bool fields in unmarshal json macro.

tests/stdlib/tjsonmacro.nim

@@ -1,3 +1,7 @@
+discard """
+  file: "tjsonmacro.nim"
+  output: ""
+"""
 import json, macros, strutils
 
 type
@@ -17,24 +21,39 @@ type
 
   Replay* = ref object
     events*: seq[ReplayEvent]
+    test: int
+    test2: string
+    test3: bool
+    testNil: string
 
-var x = Replay(
-  events: @[
-    ReplayEvent(
-      time: 1.2345,
-      kind: FoodEaten,
-      foodPos: Point[float](x: 5.0, y: 1.0)
-    )
-  ]
-)
+when isMainModule:
+  # Tests inspired by own use case (with some additional tests).
+  # This should succeed.
+  var x = Replay(
+    events: @[
+      ReplayEvent(
+        time: 1.2345,
+        kind: FoodEaten,
+        foodPos: Point[float](x: 5.0, y: 1.0)
+      )
+    ],
+    test: 18827361,
+    test2: "hello world",
+    test3: true,
+    testNil: nil
+  )
 
-let node = %x
+  let node = %x
 
-echo(node)
+  let y = to(node, Replay)
+  doAssert y.events[0].time == 1.2345
+  doAssert y.events[0].kind == FoodEaten
+  doAssert y.events[0].foodPos.x == 5.0
+  doAssert y.events[0].foodPos.y == 1.0
+  doAssert y.test == 18827361
+  doAssert y.test2 == "hello world"
+  doAssert y.test3
+  doAssert y.testNil == nil
 
-let y = to(node, Replay)
-doAssert y.events[0].time == 1.2345
-doAssert y.events[0].kind == FoodEaten
-doAssert y.events[0].foodPos.x == 5.0
-doAssert y.events[0].foodPos.y == 1.0
-echo(y.repr)
+  # Tests that verify the error messages for invalid data.
+  # TODO: