refactor: improved and generalized API plus a variety of fixes

[mccutchen]

This is an unreviewable mess, but here are the highlights:

• Revisit handshake API to make it easier to use correctly
• Split websocket server API into low-level (Read/Write) and high level (Serve) implementations
• Add connection lifecycle hooks for debugging/instrumentation
• Fix various small protocol bugs and misunderstandings
  • Only validate UTF8 on full messages, not fragments
  • Don't require masking in low level proto impl, so it can be used to read in test clients
• etc

Still lots to clean up and improve, but stopping here as a checkpoint before this gets even more unwieldy!

[codecov]

Codecov Report

Attention: Patch coverage is 76.99115% with 78 lines in your changes missing coverage. Please review.

Project coverage is 77.74%. Comparing base (bd84345) to head (2a0c663).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
proto.go	78.78%	30 Missing and 12 partials ⚠️
websocket.go	74.46%	26 Missing and 10 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #5      +/-   ##
==========================================
+ Coverage   76.36%   77.74%   +1.37%     
==========================================
  Files           1        2       +1     
  Lines         292      373      +81     
==========================================
+ Hits          223      290      +67     
- Misses         46       61      +15     
+ Partials       23       22       -1     

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