od: prefer idx_t to size_t

od: prefer idx_t to size_t

This helps find overflow bugs when compiling with -fsanitize=undefined.
* src/od.c (struct tspec, bytes_per_block, PRINT_FIELDS)
(PRINT_TYPE, print_named_ascii, print_ascii, decode_one_format)
(skip, write_block, read_block, dump, main):
Use idx_t, not size_t.