Use a rope for text storage inside of TextRun

[burg]

Right now, we simply copy all transformed text into a really long string, then put it into the TextRun. It should be possible to make a rope by transferring ownership of nonzero transformed strings, and do no copying.

[burg]

On second thought, this is not so easy.

We need to be able to do the equivalent of str::char_range_at on a Rope in order to implement TextRun::range_is_trimmable_whitespace. Currently, there is no (easy) way to start iterating characters from a specific byte index.

We also need some equivalent functionality for str::find_from, as well as iterating over all leaves specified by a byte Range. For example, this is needed to add a subset of utf8 data to the shaper buffer.

[burg]

This seems infeasible, as ropes are not sendable, and long term we want to put TextRuns into ARCs or cache them per-task. Also, ropes need some more user-friendly APIs (as noted above)

[burg]

Closing since there isn't much win to be had. Once text is shaped, its properties are queried from the GlyphStore, rather than the raw text.