refactor(layout): simplify and doc split()

[joshka]

This is mainly a reduction in density of the code with a goal of
improving mainatainability so that the algorithm is clear.

(Testing - the layout example renders the same)

[codecov]

Codecov Report

Merging #405 (0fdec45) into main (418ed20) will increase coverage by 0.73%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

❗ Current head 0fdec45 differs from pull request most recent head 1b0be84. Consider uploading reports for the commit 1b0be84 to get more accurate results

@@            Coverage Diff             @@
##             main     #405      +/-   ##
==========================================
+ Coverage   85.36%   86.09%   +0.73%     
==========================================
  Files          40       40              
  Lines        8853     9221     +368     
==========================================
+ Hits         7557     7939     +382     
+ Misses       1296     1282      -14     

Files Changed	Coverage Δ
src/backend/test.rs	43.63% <ø> (ø)
src/layout.rs	96.28% <100.00%> (+7.19%)	⬆️

[joshka]

Pinging @hasezoey for 👀 and 💭 on this.

[joshka]

Thanks for the review comments @hasezoey. I've updated the comments to better explain the testing approach (and removed a couple of comments that were accidentally left in while testing the rounding fix)

[joshka]

Incoming push makes Element one dimensional (just start, end and size()), which simplifies a bunch of this even more based @kdheepak's suggestion at #405 (comment)

I also moved the tests prior to the refactor commit to show that they apply equally to the existing code and the refactored code.

[kdheepak]

Sorry about the staged second round of comments! This looks great now btw!

[kdheepak]

LGTM!