Micro-optimize Direction

[jackkoenig]

This almost entirely API compatible, but due to ActualDirection.Bidirectional being a case class, this change breaks things like the copy method. This is mostly a performance improvement

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
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• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• API modification
• Performance improvement

Desired Merge Strategy

• Rebase (merge commit)

Release Notes

• Specified and actual direction information are each now stored as single bytes rather than references.
• This reduces the memory use of a typical bound UInt from 72 bytes shallow, 128 bytes retained to 64 bytes shallow, 120 bytes retained.
• The change is mostly source compatible, but ActualDirection.Bidirectional, has changed slightly to memoize its two possibilities (Bidirectional.Default and Bidirectional.Flipped). There are deprecations for the typical APIs

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.