feat(proof.go): adding JSON marshal/unmarshal

[distractedm1nd]

Adds json marshaling/unmarshaling to nmt.Proof.

As found elsewhere in the go stdlib, UnmarshalJSON must be defined on the pointer receiver although MarshalJSON is a value receiver.

This is needed because the fields are private. Does this need to be the case? If we make them public, JSON marshaling will work by default.

[codecov[bot]]

Codecov Report

Merging #214 (ae07a1c) into master (cdc88e8) will decrease coverage by 0.17%. The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master     #214      +/-   ##
==========================================
- Coverage   94.03%   93.86%   -0.17%     
==========================================
  Files           5        5              
  Lines         570      587      +17     
==========================================
+ Hits          536      551      +15     
- Misses         19       20       +1     
- Partials       15       16       +1     

Impacted Files	Coverage Δ
proof.go	88.48% <88.23%> (-0.03%)	:arrow_down: