refactor: unify WriteFrame/WriteFrameMasked

[mccutchen]

Also

• split MarshalFrame out of WriteFrame for easier reuse (especially in tests)
• rename CloseFrame -> NewCloseFrame for clarity
• rename public errors and their messages for more consistency

[github-actions]

🔥 Run benchmarks comparing 55ce4c0 against main:

gh workflow run bench.yaml -f pr_number=37

Note: this comment will update with each new commit.

[codecov]

Codecov Report

Attention: Patch coverage is 90.32258% with 3 lines in your changes missing coverage. Please review.

Project coverage is 88.56%. Comparing base (83b1307) to head (55ce4c0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
websocket.go	72.72%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #37      +/-   ##
==========================================
+ Coverage   88.43%   88.56%   +0.12%     
==========================================
  Files           3        3              
  Lines         441      446       +5     
==========================================
+ Hits          390      395       +5     
- Misses         37       38       +1     
+ Partials       14       13       -1     

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