TYPEMAP
Lux_IO_Btree *       T_LUX_IO_BTREE

OUTPUT
T_LUX_IO_BTREE
    XS_STRUCT2OBJ($arg, "Lux::IO::Btree", $var);

INPUT
T_LUX_IO_BTREE
    $var = XS_STATE(Lux_IO_Btree*, $arg);