Add DuplexPipe ability to do half-duplex

[AArnott]

No description provided.

[codecov]

Codecov Report

Merging #118 into master will decrease coverage by 0.1%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #118      +/-   ##
==========================================
- Coverage   89.35%   89.24%   -0.11%     
==========================================
  Files          31       31              
  Lines        2236     2260      +24     
  Branches      410      412       +2     
==========================================
+ Hits         1998     2017      +19     
- Misses        207      210       +3     
- Partials       31       33       +2

Impacted Files	Coverage Δ
src/Nerdbank.Streams/DuplexPipe.cs	90% <87.5%> (-10%)	⬇️
src/Nerdbank.Streams/MultiplexingStream.cs	89.2% <0%> (-0.48%)	⬇️

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