Use tagged pointer to represent size/length values

[nicoburns]

Objective

This is a pre-requisite for:

• Support Calc val #225
• and also Allow sum of percent and points in Dimension #229

As it allows dimension-like values (LengthPercentage, LengthPercentageAuto, Dimension, MinTrackSizingFunction, and MaxTrackSizingFunction) to contain a pointer which can be used to point to a calc() value.

Notes

• There a small-ish but non-negligible performance hit (around 10-15%) associated with this PR. That's a lot better than previous attempts at calc() and I'm assuming we'll just need to accept this. But if anyone can work out a way to avoid this then that would obviously be great.
• This requires an MSRV bump all the way up to 1.83 for const f32::to_bits
• This PR does not implement a Calc representation or the actual Calc resolution. That will be followup work.

Todo

• Fix 16 failing CSS Grid tests.
• Fix Yoga benchmark compilation

[alice-i-cecile]

There a small-ish but non-negligible performance hit (around 10-15%) associated with this PR. That's a lot better than previous attempts at calc() and I'm assuming we'll just need to accept this. But if anyone can work out a way to avoid this then that would obviously be great.

Annoying, but calc doesn't seem to be an optional feature. We could try and feature flag this, but I don't think that users are likely to want to opt out.

This requires an MSRV bump all the way up to 1.83 for const f32::to_bits

Totally fine with this: const floats will be incredibly useful to this crate in general.

This PR does not implement a Calc representation or the actual Calc resolution. That will be followup work.

Good choice!

[nicoburns]

This isn't perfect, but I think I am confident that this is the right direction. So in time-honoured Taffy tradition I think I'm going to merge and then iterate.