implemented return type inference

Other fixes:
* bind once is now the default for type classes as documented in the manual
* fixes an issue in template overloading (erroneous  ambiguity when different typedesc params were used)

tests/run/trettypeinference.nim

@@ -0,0 +1,29 @@
+discard """
+  msg:    "instantiated for string\ninstantiated for int\ninstantiated for bool"
+  output: "int\nseq[string]\nA\nB\n100\ntrue"
+"""
+
+import typetraits
+
+proc plus(a, b): auto = a + b
+
+proc `+`(a, b: string): seq[string] = @[a, b]
+
+var i = plus(10, 20)
+var s = plus("A", "B")
+
+echo i.type.name
+echo s.type.name
+
+proc inst(a): auto =
+  static: echo "instantiated for ", a.type.name
+  result = a
+
+echo inst("A")
+echo inst("B")
+echo inst(100)
+echo inst(true)
+
+# XXX: [string, tyGenericParam] is cached instead of [string, string]
+# echo inst[string, string]("C")
+