Added field 'srclen' to structure 'lua_Debug'

This new field gets the length of 'source' in the same structure.
Unlike the other strings in that structure, 'source' can be
relatively large, and Lua already has its length readily available.

lua.h

@@ -469,6 +469,7 @@ struct lua_Debug {
   const char *namewhat;	/* (n) 'global', 'local', 'field', 'method' */
   const char *what;	/* (S) 'Lua', 'C', 'main', 'tail' */
   const char *source;	/* (S) */
+  size_t srclen;	/* (S) */
   int currentline;	/* (l) */
   int linedefined;	/* (S) */
   int lastlinedefined;	/* (S) */