feat: export `VerifyLeafHashes`

[rootulp]

Closes https://github.com/celestiaorg/nmt/issues/178

[codecov[bot]]

Codecov Report

Merging #189 (0303df6) into master (1262b7f) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #189   +/-   ##
=======================================
  Coverage   95.69%   95.69%           
=======================================
  Files           5        5           
  Lines         557      557           
=======================================
  Hits          533      533           
  Misses         14       14           
  Partials       10       10           

Impacted Files	Coverage Δ
proof.go	93.70% <100.00%> (ø)