Compatibility with rust-verkle's proof format

[gballet]

Changes introduced by this PR:

• For an easier decoding of the proof, the keys and values are moved to their own field in the block. For this to happen, proof and keyvals are no longer serialized as a single block;
• It serializes the de-duplicated commitments by path, and no longer by (commitment, zi) tuples;
• It fixes some issues that got introduced with change the return type of offset2key to byte #102

[codecov-commenter]

Codecov Report

Merging #181 (84f52a2) into master (351986d) will increase coverage by 0.68%.
The diff coverage is 95.89%.

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
+ Coverage   88.30%   88.98%   +0.68%     
==========================================
  Files           9        9              
  Lines         906      908       +2     
==========================================
+ Hits          800      808       +8     
+ Misses         78       75       -3     
+ Partials       28       25       -3     

Impacted Files	Coverage Δ
proof_ipa.go	90.08% <89.28%> (+3.82%)	⬆️
config.go	100.00% <100.00%> (ø)
encoding.go	84.61% <100.00%> (+0.30%)	⬆️
stateless.go	83.57% <100.00%> (ø)
tree.go	91.57% <100.00%> (+0.21%)	⬆️

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