silent overflow of run_word_count when casting to 1 byte

[jordan-wu-97]

async PackedWrite was missing the 1-byte limiting that was in sync packed write.

capnproto-rust/capnp/src/serialize_packed.rs

Line 383 in 2ae271a

limit = in_word.offset(255);

capnproto-rust/capnp/src/serialize_packed.rs

Line 405 in 2ae271a

limit = in_ptr.offset(255 * 8);

[codecov]

Codecov Report

❌ Patch coverage is 98.30508% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 50.98%. Comparing base (ab342b3) to head (675a21e).
⚠️ Report is 267 commits behind head on master.

Files with missing lines	Patch %	Lines
capnp-futures/src/serialize_packed.rs	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #622      +/-   ##
==========================================
- Coverage   51.64%   50.98%   -0.66%     
==========================================
  Files          69       71       +2     
  Lines       33735    32479    -1256     
==========================================
- Hits        17422    16559     -863     
+ Misses      16313    15920     -393     

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[dwrensha]

Thanks!