Reduce / pass in allocations

[taj-p]

Hi!

I'm a new consumer of unicode-bidi from https://github.com/linebender/parley (integration PR).

I'm wondering what the appetite is for allowing consumers to pass in allocations to BidiInfo rather than allocations be created on a per run basis; see some example allocations:

InitialInfoExt

unicode-bidi/src/lib.rs

Lines 277 to 278 in 3bf3403

	let mut paragraphs = Vec::<ParagraphInfo>::new();
	let mut flags = Vec::<ParagraphInfoFlags>::new();

unicode-bidi/src/lib.rs

Line 310 in 3bf3403

let mut original_classes = Vec::with_capacity(text.len());

BidiInfo

unicode-bidi/src/lib.rs

Lines 505 to 506 in 3bf3403

	let mut levels = Vec::<Level>::with_capacity(text.len());
	let mut processing_classes = base.original_classes.clone();

I'm happy to try to brainstorm a solution to enable this, but am more than happy to defer to maintainer direction.

[Manishearth]

I'd be open to something like this if it can be done stably.