lua add better memory stats

lua gives you a memory used number that only reflects the current allocations
if fact it doesn't even give you a way to get the amount of ram free

rb.mem_stats() seeks to fill this gap by marking the memory allocated for lua
with a sentinel value which can later be checked to get a high water mark
of the ram used by lua and a pretty good idea of how much ram is available

Also includes an example script
usage:
used, allocd, free = rb.mem_stats()

Change-Id: Ia282869f989848324d7d88c7df4827fdbce4fb4e

apps/plugins/lua_scripts/memchk.lua

@@ -0,0 +1,32 @@
+--[[
+             __________               __   ___.
+   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+                     \/            \/     \/    \/            \/
+ $Id$
+ Example Lua Memory Use
+ Copyright (C) 2020 William Wilgus
+ This program is free software; you can redistribute it and/or
+ modify it under the terms of the GNU General Public License
+ as published by the Free Software Foundation; either version 2
+ of the License, or (at your option) any later version.
+ This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ KIND, either express or implied.
+]]--
+
+local used, allocd, free = rb.mem_stats()
+local lu = collectgarbage("count")
+local fmt = function(t, v) return string.format("%s: %d Kb\n", t, v /1024) end
+
+-- this is how lua recommends to concat strings rather than ..
+local s_t = {}
+s_t[1] = "rockbox:\n"
+s_t[2] = fmt("Used  ", used)
+s_t[3] = fmt("Allocd ", allocd)
+s_t[4] = fmt("Free  ", free)
+s_t[5] = "\nlua:\n"
+s_t[6] = fmt("Used", lu * 1024)
+s_t[7] = "\n\nNote that the rockbox used count is a high watermark"
+rb.splash_scroller(10 * rb.HZ, table.concat(s_t))