v1.18: quic: use smallvec to aggregate chunks, save 1 alloc per packet (backport of #735)

[mergify]

Use smallvec to hold chunks. Streams are packet-sized so we don't expect them to have many chunks. This saves us an allocation for each packet.

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This is an automatic backport of pull request #735 done by Mergify.

[mergify]

Cherry-pick of 55ab7fa has failed:

On branch mergify/bp/v1.18/pr-735
Your branch is up to date with 'origin/v1.18'.

You are currently cherry-picking commit 55ab7fadbc.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   Cargo.lock
	modified:   Cargo.toml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   programs/sbf/Cargo.lock
	both modified:   streamer/Cargo.toml
	both modified:   streamer/src/nonblocking/quic.rs

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.5%. Comparing base (cf5e8d3) to head (189b978).

Additional details and impacted files

@@            Coverage Diff            @@
##            v1.18     #742     +/-   ##
=========================================
- Coverage    81.5%    81.5%   -0.1%     
=========================================
  Files         827      827             
  Lines      224833   224832      -1     
=========================================
- Hits       183440   183427     -13     
- Misses      41393    41405     +12     

[steviez]

Got a little sidetracked but here is a post-merge LGTM