simple unit test and better documentation for the user defined type classes

tests/run/tusertypeclasses.nim

@@ -0,0 +1,28 @@
+discard """
+  output: "Sortable\nSortable\nContainer"
+"""
+
+import typetraits
+
+type
+  TObj = object
+    x: int
+
+  Sortable = generic x, y
+    (x < y) is bool
+
+  ObjectContainer = generic C
+    C.len is ordinal
+    for v in items(C):
+      v.type is tuple|object
+
+proc foo(c: ObjectContainer) =
+  echo "Container"
+
+proc foo(x: Sortable) =
+  echo "Sortable"
+
+foo 10
+foo "test"
+foo(@[TObj(x: 10), TObj(x: 20)])
+