New function 'lua_numbertostrbuff'

It converts a Lua number to a string in a buffer, without creating
a new Lua string.

lapi.c

@@ -368,6 +368,18 @@ LUA_API int lua_compare (lua_State *L, int index1, int index2, int op) {
 }
 
 
+LUA_API unsigned (lua_numbertostrbuff) (lua_State *L, int idx, char *buff) {
+  const TValue *o = index2value(L, idx);
+  if (ttisnumber(o)) {
+    unsigned len = luaO_tostringbuff(o, buff);
+    buff[len++] = '\0';  /* add final zero */
+    return len;
+  }
+  else
+    return 0;
+}
+
+
 LUA_API size_t lua_stringtonumber (lua_State *L, const char *s) {
   size_t sz = luaO_str2num(s, s2v(L->top.p));
   if (sz != 0)