feat!: checks the exact size of the node in HashNode

[staheri14]

Overview

Closes #153 (part of https://github.com/celestiaorg/nmt/issues/173)

Checklist

• [x] New and updated code has appropriate documentation
• [x] New and updated code has new and/or updated testing
• [x] Required CI checks are passing
• [x] Visual proof for any user facing features like CLI or documentation updates
• [x] Linked issues closed with keywords

[codecov[bot]]

Codecov Report

Merging #175 (dd6a640) into master (2c15503) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #175   +/-   ##
=======================================
  Coverage   95.76%   95.76%           
=======================================
  Files           5        5           
  Lines         519      519           
=======================================
  Hits          497      497           
  Misses         13       13           
  Partials        9        9           

Impacted Files	Coverage Δ
hasher.go	97.54% <100.00%> (ø)

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[staheri14]

this change seems like it could be consensus breaking if some nodes use it and others don't, so we might want to rename this PR to feat!... to indicate such a change

Right, updated the PR description.