x/text/unicode/bidi: consider reusing internal slices

[benoitkugler]

Go version

go version 1.22

Output of go env in your module/workspace:

Non relevant

What did you do?

In go-text/typesetting, we use bidi to segment the input text. We store and reuse one bidi.Paragraph object, calling its SetString method for each new input.

What did you see happen?

The bidi.Paragraph.prepareInput method has high memory allocation.

What did you expect to see?

Only few allocations. If possible, the slices pairTypes, pairValues, types should reuse their capacity (instead of being freed and immediately re-allocated).

On the same topic, paragraph.determineLevelRuns allocates a new run []int slice, but it is perhaps harder to get rid of this allocation.

[mauri870]

Friendly ping @mpvl per https://dev.golang.org/owners