Work towards maildir sorting, and Lua integration.

* Add some temporary timing code to compare sort methods.
* Add a different way of using Lua to sort maildirs - instead of calling
a Lua function for each comparison, call a Lua function for each maildir
once, returning a string key which is used directly in std::sort.  This
much faster than the O(n*log(n)) calls for comparison, though maybe a bit
less natural.
* Rather than defining separate CLua::filter, CLua::compare, etc., experiment
with a variadic template:
template <typename Ret, typename... Params>
Ret call(lua_State *L, const char *name, Params... params);