use AsRef instead of Deref in NoAllocBufferSegments

[dwrensha]

This follows the pattern in #474.

Note in particular that [u8; N] implements AsRef<[u8]> but does not implement Deref<Target=[u8]>.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (857b324) 51.85% compared to head (b5b7b7d) 51.80%.

Files	Patch %	Lines
capnp/src/serialize/no_alloc_buffer_segments.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #475      +/-   ##
==========================================
- Coverage   51.85%   51.80%   -0.05%     
==========================================
  Files          69       69              
  Lines       34137    34137              
==========================================
- Hits        17702    17685      -17     
- Misses      16435    16452      +17     

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