next steps towards term rewriting macros; simple examples work

tests/run/tpatterns.nim

@@ -0,0 +1,17 @@
+discard """
+  output: '''48
+hel'''
+"""
+
+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)
+
+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]