Move line numbers out of measured lines

[io7m]

Bounds in a measured line should be returned in text-local space (not parent-relative space).

Line numbers should be moved out of the measured line data structure.

With these two changes, text models can avoid recalculating measured lines just because some earlier text section was changed and now all of the line numbers are incorrect. Line numbers should be a property of the structures storing the measured lines, not the measured lines themselves.

[io7m]

Done.