Properly size the destination in flatten_segments

[bobrik]

We've observed unexpected reallocations on some flamegraphs here.

Looks like it stems from 5afd805. cc @dwrensha

[bobrik]

Perhaps the right thing to do is to put / BYTES_PER_WORD into compute_serialized_size_in_words where it would match the label and leave compute_serialized_size and flatten_segments without it. Let me know what you think.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 50.65%. Comparing base (ab342b3) to head (1a2c816).
⚠️ Report is 198 commits behind head on master.

Files with missing lines	Patch %	Lines
capnp/src/serialize.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #590      +/-   ##
==========================================
- Coverage   51.64%   50.65%   -0.99%     
==========================================
  Files          69       70       +1     
  Lines       33735    32331    -1404     
==========================================
- Hits        17422    16377    -1045     
+ Misses      16313    15954     -359     

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[dwrensha]

Good catch! Thanks! I'll touch this up a bit...

[dwrensha]

I implemented your suggestion in 41f933c.