performance: Appends to region_highlight are quadratic?

[danielshahaf]

@psprint reports that appending to $region_highlight is faster if first accumulating to another array and then appending to $region_highlight in one shot, compared to appending to $region_highlight directly.

Speculation: this may be because the $region_highlight is a special array (in the ${(t)foo} sense), and because the array's setfn is called on the entire array each time, not only on the appended portion.

• Investigate the difference quantitatively
• If needed, switch from appending directly to using a temporary array

[danielshahaf]

workers/39869 might be related.

[danielshahaf]

If needed, switch from appending directly to using a temporary array

Note that array appends are quadratic too, independently of region_highlight's setter. Possibly with a smaller big-O constant.