Finish up Purge Mode:

o Make it recognize constant expressions (yay!)
o Make it not add a blank line to end of *.purged files.
o Make it nuke #!P comments in the source files.

src/types.h

@@ -96,6 +96,7 @@ typedef struct {
     DynamicBuffer pushedToken;	/* Pushed-back token */
     char const *tokenPushed;	/* NULL if no pushed-back token */
     char expr_happened;         /* Did we encounter an [expression] ? */
+    char nonconst_expr;         /* Did we encounter a non-constant [expression] ? */
 } Parser;
 
 typedef Parser *ParsePtr;  /* Pointer to parser structure */