better overloading resolution for generics

tests/run/toverl3.nim

@@ -0,0 +1,20 @@
+discard """
+  file: "toverl3.nim"
+  output: '''m1
+tup1'''
+"""
+
+# Tests more specific generic match:
+
+proc m[T](x: T) = echo "m2"
+proc m[T](x: var ref T) = echo "m1"
+
+proc tup[S, T](x: tuple[a: S, b: ref T]) = echo "tup1"
+proc tup[S, T](x: tuple[a: S, b: T]) = echo "tup2"
+
+var
+  obj: ref int
+  tu: tuple[a: int, b: ref bool]
+  
+m(obj)
+tup(tu)