Reduce copies in frame encoding and segment uploads

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[codecov]

Codecov Report

❌ Patch coverage is 98.55556% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.23%. Comparing base (bf627b1) to head (c3c3719).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
zerofs/src/object_store_prefetch.rs	94.78%	6 Missing ⚠️
zerofs/src/ninep/handler.rs	91.66%	3 Missing ⚠️
zerofs/src/fs/store/extent/read.rs	98.64%	2 Missing ⚠️
zerofs/src/segment.rs	98.54%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #630      +/-   ##
==========================================
+ Coverage   82.91%   83.23%   +0.32%     
==========================================
  Files         103      104       +1     
  Lines       46415    47003     +588     
==========================================
+ Hits        38483    39122     +639     
+ Misses       7932     7881      -51     

Files with missing lines	Coverage Δ
zerofs/src/frame_codec.rs	100.00% <100.00%> (ø)
zerofs/src/fs/ops/io.rs	97.75% <100.00%> (+<0.01%)	⬆️
zerofs/src/fs/ops/setattr.rs	81.24% <100.00%> (ø)
zerofs/src/fs/store/extent/mod.rs	96.29% <100.00%> (+5.09%)	⬆️
zerofs/src/fs/store/extent/test_util.rs	96.77% <100.00%> (+0.04%)	⬆️
zerofs/src/fs/store/extent/write.rs	94.32% <100.00%> (+1.93%)	⬆️
zerofs/src/ninep/response.rs	100.00% <100.00%> (ø)
zerofs/src/ninep/server.rs	93.65% <100.00%> (+0.52%)	⬆️
zerofs/src/segment_store.rs	90.16% <100.00%> (+0.41%)	⬆️
zerofs/src/fs/store/extent/read.rs	88.82% <98.64%> (+5.31%)	⬆️
... and 3 more

... and 4 files with indirect coverage changes

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