fix endianness issues in commitment calculations

[gballet]

The endianness in commitments was wrong, this PR fixes it.

[gballet]

failing tests: looks like the endianness problem is also present in the proof, which is to be expected looking at the multipoint part.

[codecov-commenter]

Codecov Report

Merging #185 (7d8046c) into master (150c713) will decrease coverage by 0.57%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #185      +/-   ##
==========================================
- Coverage   88.64%   88.06%   -0.58%     
==========================================
  Files           9        9              
  Lines         916      905      -11     
==========================================
- Hits          812      797      -15     
- Misses         79       82       +3     
- Partials       25       26       +1     

Impacted Files	Coverage Δ
proof_ipa.go	89.18% <ø> (-1.22%)	⬇️
ipa.go	100.00% <100.00%> (ø)
stateless.go	83.57% <100.00%> (ø)
tree.go	90.68% <100.00%> (-0.89%)	⬇️

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