Internalized string "break" kept by the parser

The parser uses "break" as fake label to compile "break" as "goto
break". To avoid producing this string at each use, it keeps it
available in its state.

llex.h

@@ -75,6 +75,7 @@ typedef struct LexState {
   struct Dyndata *dyd;  /* dynamic structures used by the parser */
   TString *source;  /* current source name */
   TString *envn;  /* environment variable name */
+  TString *brkn;  /* "break" name (used as a label) */
 } LexState;