Generate field number constants.  Patch from Michael Poole.

CONTRIBUTORS.txt

diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index 5c6d9d2..0ad37d9 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -62,3 +62,4 @@
   Michael Poole <mdpoole@troilus.org>
     * Fixed warnings about generated constructors not explicitly initializing
       all fields (only present with certain compiler settings).
+    * Added generation of field number constants.