implemented 'let' statement

tests/run/tlet.nim

@@ -0,0 +1,19 @@
+discard """
+  output: '''Very funny, your name is name.
+nameabc'''
+"""
+
+proc main =
+  let name = "name"
+  if name == "":
+    echo("Poor soul, you lost your name?")
+  elif name == "name":
+    echo("Very funny, your name is name.")
+  else:
+    Echo("Hi, ", name, "!")
+    
+  let (x, y) = ("abc", name)
+  echo y, x
+
+main()
+