Increase the obarray size.

In a typical GNU/Linux/X11 build, we wind up with over 15k symbols by
the time we've started.  The old obarray size ensured an average chain
length of 10 or more.

* src/lread.c (OBARRAY_SIZE): Increase to 15121.

src/lread.c

@@ -4116,7 +4116,7 @@ OBARRAY defaults to the value of `obarray'.  */)
   return Qnil;
 }
 
-#define OBARRAY_SIZE 1511
+#define OBARRAY_SIZE 15121
 
 void
 init_obarray (void)