Customize Lua GC, add object-memory-lock API.

Add custom API to prevent Lua Objects from being garbage collected.
This is intended to for Ardour LuaBridge bindings (~1MB Objects:
tables, functions and userdata).
The bindings are persistent and the gc can skip them in mark & sweep
phases. This is a significant performance improvement for garbage
collection.

Note. The next version of Lua (5.4) will come with a generational-gc
rather than an incremental, so extending the API at this point in time
is acceptable.

libs/lua/lua-5.3.4/lua.h

@@ -311,6 +311,8 @@ LUA_API int (lua_isyieldable) (lua_State *L);
 
 LUA_API int (lua_gc) (lua_State *L, int what, int data);
 
+LUA_API void (lua_mlock) (lua_State *L, int enable);
+
 
 /*
 ** miscellaneous functions