cut down textprop size a bit... 72 -> 64bytes. if we can wrap tp->start/len/text_offset/text_len read/wrtie in access funcs. so we can special case where:
- start == text_offset == 0 && len == text_len == 1
- start == text_offset == 0 && len == text_len < 65536
- start == text_offset == 0 && len == text_len < 256