bit_array: Simplify subtraction

[ValarDragon]

This significantly simplifies subtraction logic. I suspect it also significantly speeds it up due to defer removal.

• Updated all relevant documentation in docs - n/a
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGELOG_PENDING.md - seems like a fairly internal change

[ValarDragon]

I've been doing too much algebra over finite field extensions lol. I got the carry-less subtraction confused with over there where it is an xor. It should actually be an and not. Changed that in the latest commit, tests should now pass.

[codecov-io]

Codecov Report

Merging #2533 into develop will decrease coverage by 0.08%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #2533      +/-   ##
===========================================
- Coverage    61.36%   61.27%   -0.09%     
===========================================
  Files          202      202              
  Lines        16731    16681      -50     
===========================================
- Hits         10267    10222      -45     
+ Misses        5598     5592       -6     
- Partials       866      867       +1

Impacted Files	Coverage Δ
libs/common/bit_array.go	76.62% <100%> (-2.88%)	⬇️
libs/common/math.go	31.25% <0%> (-12.5%)	⬇️
libs/db/remotedb/remotedb.go	36.84% <0%> (-4.69%)	⬇️
libs/autofile/autofile.go	75% <0%> (-2.64%)	⬇️
consensus/reactor.go	72.6% <0%> (-0.65%)	⬇️
consensus/state.go	77.3% <0%> (-0.24%)	⬇️

[melekes]

🍰 🌮 🍉

Great work!