Mention computing Dragon Curve parity in constant time

[ebblake]

Description

The existing O(log n) solution solves in under 1µs, so there is not
really much to be shaved by swapping to an O(1) computation per bit.
But it is still a fun read, so worth documenting for anyone else using
this repository as a reference point.

It is also worth adding a unit test of the example given in part 1.

Type of change

• Performance improvement
• Bug fix
• Other (docs, unit test)

Checklist

• Pull request title and commit messages are clear and informative.
• Documentation has been updated if necessary.
• Code style matches the existing code. This one is somewhat subjective, but try to "fit in" by
  using the same naming conventions. Code should be portable, avoiding any architecture
  specific intrinsics.
• Tests pass cargo test
• Code is formatted cargo fmt -- `find . -name "*.rs"`
• Code is linted cargo clippy --all-targets --all-features

Formatting and linting also can be executed by running just
(if installed) on the command line at the project root.

[ebblake]

https://www.reddit.com/r/adventofcode/comments/1r642oc/2016_day_16_in_review_dragon_checksum/ mentions OEIS A255070 as a source for a similar derivation of O(1) Dragon parity

[maneatingape]

The existing nlog(n) solution is already under 1µs so my preference is to keep it, as it's easily explainable and understandable.

However askalskis post is an interesting read (especially I have no idea why it works, only that it does work. 😀 )
so how about we take the same approach as 2021 Day 19 and reduce this PR to a short comment linking to the post.

[ebblake]

The existing nlog(n) solution is already under 1µs so my preference is to keep it, as it's easily explainable and understandable.

Fair enough.

However askalskis post is an interesting read (especially I have no idea why it works, only that it does work. 😀 ) so how about we take the same approach as 2021 Day 19 and reduce this PR to a short comment linking to the post.

Patch revised in place. I did, however, salvage the unit test addition out of my bigger patch.

[maneatingape]

Thanks for adding the test!