Move VHDL global state management to a single file

The new state.cc/hh file now manages all the global
state that we maintain while generating VHDL. This
should make the code a bit tidier.

tgt-vhdl/expr.cc

@@ -20,6 +20,7 @@
 
 #include "vhdl_target.h"
 #include "support.hh"
+#include "state.hh"
 
 #include <iostream>
 #include <cassert>