improvements to get code size down for programs that don't use GC

tests/reject/tprocvar.nim

@@ -0,0 +1,18 @@
+discard """
+  errormsg: "type mismatch"
+  line: 17
+  file: "tprocvar.nim"
+"""
+
+type
+  TCallback = proc (a, b: int)
+
+proc huh(x, y: var int) =
+  x = 0
+  y = x+1
+
+proc so(c: TCallback) =
+  c(2, 4)
+  
+so(huh)
+