long strings are created directly in final position when possible

(instead of using an auxiliar buffer to first create the string
and then allocate the final string and copy result there)

lstate.h

@@ -1,5 +1,5 @@
 /*
-** $Id: lstate.h,v 2.122 2015/06/01 16:34:37 roberto Exp roberto $
+** $Id: lstate.h,v 2.123 2015/07/04 16:33:17 roberto Exp roberto $
 ** Global State
 ** See Copyright Notice in lua.h
 */
@@ -131,7 +131,6 @@ typedef struct global_State {
   GCObject *tobefnz;  /* list of userdata to be GC */
   GCObject *fixedgc;  /* list of objects not to be collected */
   struct lua_State *twups;  /* list of threads with open upvalues */
-  Mbuffer buff;  /* temporary buffer for string concatenation */
   unsigned int gcfinnum;  /* number of finalizers to call in each GC step */
   int gcpause;  /* size of pause between successive GCs */
   int gcstepmul;  /* GC 'granularity' */