next steps towards term rewriting macros; simple examples work

tests/reject/tdisallowif.nim

@@ -0,0 +1,28 @@
+discard """
+  line: 24
+  errormsg: "usage of 'disallowIf' is a user-defined error"
+"""
+
+template optZero{x+x}(x: int): int = x*3
+template andthen{x*3}(x: int): int = x*4
+template optSubstr1{x = substr(x, a, b)}(x: string, a, b: int) = setlen(x, b+1)
+
+template disallowIf{
+  if cond: action
+  else: action2
+}(cond: bool, action, action2: stmt) {.error.} = action
+
+var y = 12
+echo y+y
+
+var s: array[0..2, string]
+s[0] = "hello"
+s[0] = substr(s[0], 0, 2)
+
+echo s[0]
+
+if true:
+  echo "do it"
+  echo "more branches"
+else:
+  nil