Implement ZIP 302 structured memos

[str4d]

Implements zcash/zips#638.

[str4d]

Force-pushed to fix a bug and CI lints.

[str4d]

IIRC I copied this type code from some existing implementation of structured memos. It was Sapling-specific then, but we can easily expand its definition to be any valid recipient address encoding.

Note that this uses a byte encoding rather than a string encoding for more efficient packing, so we may need to consider how this interacts with Unified Addresses (which now exist). We don't need to serialize the network of the address, as that is implicit in the transaction (via the chain in which it is mined), so can just be provided by the caller if they want to take the address and encode it as a string.

[codecov]

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 ⚠️
zcash_primitives/src/memo/builder.rs	0.00%	17 Missing ⚠️
zcash_primitives/src/memo.rs	18.18%	9 Missing ⚠️

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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