Deep userdata changes

* bumped version to 3.9.6
* separate deep userdata code in a dedicated file to allow external
modules to implement Lanes-compatible deep userdata without requiring a
binary dependency against the Lanes module. because of this linda_id
function(eDO_metatable) must push 2 values on the stack: a metatable and
a deep version string obtained from luaG_pushdeepversion()