Implement ZIP 302 structured memos

[str4d]

Implements zcash/zips#638.

Closes #2234.
Closes COR-1121.

[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

❌ Patch coverage is 24.60317% with 95 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.59%. Comparing base (32f8ad9) to head (3114723).
⚠️ Report is 419 commits behind head on main.

Files with missing lines	Patch %	Lines
components/zcash_protocol/src/memo/structured.rs	29.89%	68 Missing ⚠️
components/zcash_protocol/src/memo/builder.rs	0.00%	18 Missing ⚠️
components/zcash_protocol/src/memo.rs	18.18%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #668      +/-   ##
==========================================
- Coverage   58.74%   58.59%   -0.16%     
==========================================
  Files         208      210       +2     
  Lines       27748    27873     +125     
==========================================
+ Hits        16300    16331      +31     
- Misses      11448    11542      +94     

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[str4d]

Rebased on main. This PR still needs to be rewritten to match what is proposed in zcash/zips#638 (written a year ago but decided upon three years ago, three years after this PR was first implemented).