unboxing vector

let unbox_vec llval =
  let ptr = build_in_bounds_gep llval (idx 3) "boxptr" builder in
  let size_ptr = build_in_bounds_gep llval (idx 5) "sizeptr" builder in
  let size = build_load size_ptr "size" builder in
  let el = build_load ptr "el" builder in
  let rhvector_type size = pointer_type (vector_type i64_type size) in
  let vec n = build_bitcast el (rhvector_type n) "vecptr" builder in
  let vecbb n = block_of_value (vec n) in
  let llsize n = const_int i32_type n in
  let switch = build_switch size (vecbb 1) 10 builder in
  let add_case_vec n = add_case switch (llsize n) (vecbb n) in
  add_case_vec 1; add_case_vec 2; add_case_vec 3;
  switch