Implement a stateless version of the verkle tree

[gballet]

This PR implements a new type of node, meant to create a tree from elements of a proof.

[gballet]

Failing tests are probably due to the modification of a global constant...

[codecov-commenter]

Codecov Report

Merging #144 (aea74d4) into master (7f86b3f) will decrease coverage by 0.14%.
The diff coverage is 88.38%.

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
- Coverage   89.48%   89.33%   -0.15%     
==========================================
  Files           8        9       +1     
  Lines         666      863     +197     
==========================================
+ Hits          596      771     +175     
- Misses         49       66      +17     
- Partials       21       26       +5     

Impacted Files	Coverage Δ
tree.go	90.45% <0.00%> (ø)
stateless.go	88.71% <88.71%> (ø)
ipa.go	100.00% <100.00%> (ø)

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