Flatten "size" ParseNode and also simplify its uses.

[marcianx]

Towards #1492.

[codecov]

Codecov Report

Merging #1554 into master will increase coverage by 0.05%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master    #1554      +/-   ##
==========================================
+ Coverage   84.12%   84.17%   +0.05%     
==========================================
  Files          79       79              
  Lines        4554     4551       -3     
  Branches      803      803              
==========================================
  Hits         3831     3831              
+ Misses        628      625       -3     
  Partials       95       95

Impacted Files	Coverage Δ
src/functions/cr.js	95.65% <ø> (ø)	⬆️
src/Parser.js	93.94% <ø> (ø)	⬆️
src/parseNode.js	84.21% <ø> (ø)	⬆️
src/functions/rule.js	95.23% <ø> (ø)	⬆️
src/environments/array.js	45.49% <0%> (ø)	⬆️
src/functions/kern.js	81.25% <100%> (ø)	⬆️
src/functions/raisebox.js	100% <100%> (ø)	⬆️
src/functions/genfrac.js	89.32% <66.66%> (+1.48%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5b6ffd7...bb07401. Read the comment docs.