Saves 45 bytes in fileioc

Changes:

• util_ret_null_pop_ix was only used by one function, so I moved it inside that function so it could be reached by jr instead of jp
• ld iy, 0 \ add iy, sp \ ... was changed to ld hl, 3 \ add hl, sp \ ... when no more than 2 arguments are to be retrieved from the stack. This saves at 1 byte + 1F when retrieving 2 24bit arguments for example.
• util_set_offset was tailed called by one function, so I moved it to that function to remove a jp.
• util_ret_neg_one does scf \ sbc hl, hl \ ret. It just so happens that ti_Delete also contains the instructions scf \ sbc hl, hl \ ret, so I moved util_ret_neg_one to the end of ti_Delete, meaning that util_ret_neg_one now takes up zero bytes of space. I did something similar with util_ret_null as well.
• A few other minor optimizations were done, like removing the double ex de, hl in ti_Read and using inc.s bc to clear UBC instead of ld bc, 0 in util_is_slot_open.