'is' operator for generic code

tests/accept/run/tisopr.nim

@@ -0,0 +1,13 @@
+discard """
+  output: "true true false yes"
+"""
+
+proc IsVoid[T](): string = 
+  when T is void:
+    result = "yes"
+  else:
+    result = "no"
+
+const x = int is int
+echo x, " ", float is float, " ", float is string, " ", IsVoid[void]()
+