utfu-compatibility.py: add comments, don’t fill the attributes of the first code point of a range twice, don’t read the entire UnicodeData.txt file at once in fill_attributes()

See:

3ccaf53dd71a6d927d55eeb03fb08d26af3d4534
83cd925026093ebae4e0b7802b709b98e05c1227
7ef5161898f54d2d66bb25f898310c68bc2d6577