[POC] Shaper based itemisation

[taj-p]

See linebender#462 (comment)

To do:

• inline hole detection during cluster assignment (instead of paying redundant iteration)
• consider not using temporary allocations for shaped segments and holes (before pushing them to ultimate data structure)
• lots of review / addressing inline to dos

[taj-p]

This removes most of the normalisation work from Parley

[taj-p]

The changes in this file allow for shaped segments and holes to be stored in separate structures before deciding how they should be pushed to the Layout. I think a mature implementation will remove these temporary data structures and mutate Layout directly (to avoid redundant copies).

[taj-p]

This file can be ignored. By shaping with more fonts, the need to cache generating font metrics increases. So I added this LRU cache on font metrics to get a feel for what the ultimate performance of this approach could be.

[taj-p]

This file contains a lot of refactoring and handling of shaped segments / holes.