Use rlang::hash instead of digest

[thomasp85]

Fix #4458

This PR removes the digest dependencies in favour of rlang::hash().

Since guides are sorted by their hash value and the hash value changes it can lead to changes in the automatic ordering of legends, as reflected in the change in visual tests. Since we make no promises on how they are sorted I think this is fine but should of course be communicated

[yutannihilation]

Looks good, and I agree with you on this point,

Since we make no promises on how they are sorted

however, IIUC, the legend order had been consistent for this 11 years (since the this first commit?: 0795c6f), which sounds long enough to make the users assume the behavior would never change. So, I think the NEWS item should have a bit more words (probably including how to determine the order manually by guide_*(order = )?), although this change isn't so "breaking"

[thomasp85]

@yutannihilation I've improved the news bullet - it will be part of the "Breaking Changes" section in the news once we polish it prior to release

[yutannihilation]

Thanks, looks good!