make field length optional for maps

[vedhavyas]

No description provided.

[codecov]

Codecov Report

Merging #97 (14bb51c) into master (2ed0cc3) will decrease coverage by 2.45%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
- Coverage   89.62%   87.17%   -2.46%     
==========================================
  Files           3        3              
  Lines         810      741      -69     
==========================================
- Hits          726      646      -80     
- Misses         46       55       +9     
- Partials       38       40       +2     

Impacted Files	Coverage Δ
proofs/property.go	98.52% <ø> (-0.16%)	⬇️
proofs/flatten.go	82.44% <33.33%> (-3.63%)	⬇️
proofs/tree.go	87.06% <0.00%> (-1.90%)	⬇️

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