moved meson up the dependency chain

--- a/src/HOL/Meson.thy	Thu Nov 21 21:33:34 2013 +0100
+++ b/src/HOL/Meson.thy	Thu Nov 21 21:33:34 2013 +0100
@@ -8,7 +8,7 @@
 header {* MESON Proof Method *}
 
 theory Meson
-imports Datatype
+imports Nat
 begin
 
 subsection {* Negation Normal Form *}