Add NetConn adapter and protect against Reader after CloseRead

[nhooyr]

Closes #100
Closes #101

[codecov]

Codecov Report

Merging #102 into master will decrease coverage by 0.91%.
The diff coverage is 56.6%.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
- Coverage   75.86%   74.95%   -0.92%     
==========================================
  Files          13       14       +1     
  Lines         986     1038      +52     
==========================================
+ Hits          748      778      +30     
- Misses        184      202      +18     
- Partials       54       58       +4

Impacted Files	Coverage Δ
websocket.go	70.27% <0%> (-0.46%)	⬇️
netconn.go	61.22% <61.22%> (ø)

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