implemented 'let' statement

tests/reject/tlet.nim

@@ -0,0 +1,11 @@
+discard """
+  line: "10"
+  errormsg: "'name' cannot be assigned to"
+"""
+
+Echo("What's your name? ")
+let name = readLine(stdin)
+while name == "":
+  Echo("Please tell me your name: ")
+  name = readLine(stdin)
+

tests/reject/tlet2.nim

@@ -0,0 +1,16 @@
+discard """
+  line: "13"
+  errormsg: "for a 'var' type a variable needs to be passed"
+"""
+
+proc divmod(a, b: int, res, remainder: var int) =
+  res = a div b        # integer division
+  remainder = a mod b  # integer modulo operation
+
+let
+  x = 9
+  y = 3
+divmod(8, 5, x, y) # modifies x and y
+echo(x)
+echo(y)
+