While working on something else, I noticed that tdesc_data_cleanup
took a void* parameter. Looking more into this, I found that
tdesc_use_registers expected a transfer of ownership.
I think it's better to express this sort of thing via the type system,
when possible. This patch changes tdesc_data_alloc to return a unique
pointer, changes tdesc_use_registers to accept an rvalue reference,
and then adapts all the users.
Note that a deleter structure is introduced to avoid having to move
tdesc_arch_data to the header file.