bugfix: 'indexOf' for tuple fields works

tests/run/tfieldindex.nim

@@ -0,0 +1,21 @@
+discard """
+  output: "1"
+"""
+
+type
+  TMyTuple = tuple[a, b: int]
+
+proc indexOf*(t: typedesc, name: string): int {.compiletime.} =
+  ## takes a tuple and looks for the field by name.
+  ## returs index of that field.
+  var
+    d: t
+    i = 0
+  for n, x in fieldPairs(d):
+    if n == name: return i
+    i.inc
+  raise newException(EInvalidValue, "No field " & name & " in type " & 
+    astToStr(t))
+
+echo TMyTuple.indexOf("b")
+