Ensure leafs are 32-bytes

[fasmat]

If a merkle tree is built with leafs that aren't 32 bytes long ValidatePartialTree and other methods will fail.

I added a check to AddLeafs to ensure that only values with 32 byte length are added to the tree.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.97%. Comparing base (e05293e) to head (5bdb848).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
merkle.go	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #51      +/-   ##
===========================================
- Coverage    78.45%   77.97%   -0.48%     
===========================================
  Files           12       12              
  Lines          724      722       -2     
===========================================
- Hits           568      563       -5     
- Misses         132      134       +2     
- Partials        24       25       +1     

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[noamnelke]

lgtm