Tidy up note package

[AlCutter]

This PR tidies up the note package a bit:

• Move "common" structs and errors into a new note.go file
• Add/update top-level NewSigner and NewVerifier functions to natively handle ed25519 keys via the local Signer interface which gives access to the associated verifier.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.00000% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.85%. Comparing base (fa00c16) to head (24914c6).
⚠️ Report is 182 commits behind head on main.

Files with missing lines	Patch %	Lines
note/note.go	75.00%	10 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #271      +/-   ##
==========================================
- Coverage   82.57%   78.85%   -3.73%     
==========================================
  Files           5       10       +5     
  Lines         241      922     +681     
==========================================
+ Hits          199      727     +528     
- Misses         30      108      +78     
- Partials       12       87      +75     

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