Allow ReaderSegments to be implemented for references to unsized ReaderSegments

[ypoluektovich]

Currently &[&[u8]] does not implement ReaderSegments despite [&[u8]] implementing it, because [&[u8]] is not Sized. SegmentArray exists to overcome this, but there's doesn't seem to be a need for forcing Sized, as all methods of ReaderSegments accept Self via a reference.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.38%. Comparing base (ab342b3) to head (dacf631).
Report is 126 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #546      +/-   ##
==========================================
+ Coverage   51.64%   52.38%   +0.73%     
==========================================
  Files          69       70       +1     
  Lines       33735    34587     +852     
==========================================
+ Hits        17422    18117     +695     
- Misses      16313    16470     +157     

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[dwrensha]

Thanks!