Add untokenize() function to allow full round-trip tokenization.

Should significantly enhance the utility of the module by supporting
the creation of tools that modify the token stream and writeback the
modified result.
diff --git a/Lib/test/regrtest.py b/Lib/test/regrtest.py
index 6160b3d..85e784b 100755
--- a/Lib/test/regrtest.py
+++ b/Lib/test/regrtest.py
@@ -91,7 +91,9 @@
 
     compiler -  Test the compiler package by compiling all the source
                 in the standard library and test suite.  This takes
-                a long time.
+                a long time.  Enabling this resource also allows
+                test_tokenize to verify round-trip lexing on every
+                file in the test library.
 
     subprocess  Run all tests for the subprocess module.