Update ZIP 321 parsing to use the MemoBytes type.

[nuttycom]

This also adds documentation for the zip321 module.

[codecov]

Codecov Report

Merging #362 (dd15e5f) into master (44e3176) will increase coverage by 0.37%.
The diff coverage is 83.33%.

❗ Current head dd15e5f differs from pull request most recent head f30502c. Consider uploading reports for the commit f30502c to get more accurate results

@@            Coverage Diff             @@
##           master     #362      +/-   ##
==========================================
+ Coverage   64.49%   64.86%   +0.37%     
==========================================
  Files          70       70              
  Lines        7176     7152      -24     
==========================================
+ Hits         4628     4639      +11     
+ Misses       2548     2513      -35     

Impacted Files	Coverage Δ
zcash_client_backend/src/zip321.rs	70.14% <83.33%> (+5.90%)	⬆️
zcash_primitives/src/memo.rs	47.36% <0.00%> (+8.77%)	⬆️

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