Changed the growth rate of string buffers

The growth rate of string buffers was reduced from 2 to 1.5 (3/2).
As string buffers start larger (256~1024 bytes), they don't need to
grow that fast. Moreover, a lower rate allows multiplicative growth up
to larger sizes (3/2 of the maximum). (After that, the growth becomes
linear, which is mostly useless.)

luaconf.h

@@ -747,14 +747,15 @@
 
 /*
 @@ LUA_IDSIZE gives the maximum size for the description of the source
-@@ of a function in debug information.
+** of a function in debug information.
 ** CHANGE it if you want a different size.
 */
 #define LUA_IDSIZE	60
 
 
 /*
-@@ LUAL_BUFFERSIZE is the buffer size used by the lauxlib buffer system.
+@@ LUAL_BUFFERSIZE is the initial buffer size used by the lauxlib
+** buffer system.
 */
 #define LUAL_BUFFERSIZE   ((int)(16 * sizeof(void*) * sizeof(lua_Number)))