Implement ZIP 302 structured memos

[str4d]

Implements https://github.com/zcash/zips/pull/638.

[str4d]

Force-pushed to fix a bug and CI lints.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 25.75758% with 98 lines in your changes missing coverage. Please review.

Project coverage is 75.08%. Comparing base (e940620) to head (e7a12ea). Report is 1352 commits behind head on main.

Files	Patch %	Lines
zcash_primitives/src/memo/structured.rs	30.76%	72 Missing :warning:
zcash_primitives/src/memo/builder.rs	0.00%	17 Missing :warning:
zcash_primitives/src/memo.rs	18.18%	9 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #668      +/-   ##
==========================================
- Coverage   75.70%   75.08%   -0.63%     
==========================================
  Files         103      105       +2     
  Lines       10535    10666     +131     
==========================================
+ Hits         7976     8009      +33     
- Misses       2559     2657      +98     

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