Add UsePipeReader and UsePipeWriter extension methods on Stream

[AArnott]

This allows accessing a stream using the pipe API.

These extension methods perform the opposite function to the PipeStream class, which converts pipe reader/writer into a Stream.

[codecov]

Codecov Report

Merging #11 into master will decrease coverage by 1.58%.
The diff coverage is 85.39%.

@@            Coverage Diff             @@
##           master      #11      +/-   ##
==========================================
- Coverage   91.34%   89.75%   -1.59%     
==========================================
  Files          12       12              
  Lines         820      908      +88     
  Branches       91      100       +9     
==========================================
+ Hits          749      815      +66     
- Misses         46       61      +15     
- Partials       25       32       +7

Impacted Files	Coverage Δ
src/Nerdbank.Streams/StreamExtensions.cs	85.55% <85.39%> (-14.45%)	⬇️
src/Nerdbank.Streams/MultiplexingStream.Channel.cs	92.3% <0%> (-5.13%)	⬇️
src/Nerdbank.Streams/FullDuplexStream.cs	98.19% <0%> (-1.81%)	⬇️
src/Nerdbank.Streams/MultiplexingStream.cs	80.24% <0%> (-1.58%)	⬇️

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