Defragment incoming stream chunks (fixes #431)

[djc]

Here's an attempt at solving that old memory fragmentation issue. The 32 threshold is just a simple heuristic: in the worst case we accept 32 1-byte chunks from 32 different datagrams, 32k of unnecessary allocations, but hopefully most applications would not get up to 32 chunks in the buffer so that they would not trigger the (relatively slow) defragmentation code.

Let me know what you think!

[codecov]

Codecov Report

Merging #598 into master will decrease coverage by 2.67%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master     #598      +/-   ##
==========================================
- Coverage   69.14%   66.46%   -2.68%     
==========================================
  Files          64       64              
  Lines       11120    11637     +517     
==========================================
+ Hits         7689     7735      +46     
- Misses       3431     3902     +471

Impacted Files	Coverage Δ
quinn-proto/src/assembler.rs	97.05% <96.29%> (-0.19%)	⬇️
quinn-h3/src/qpack/prefix_string/encode.rs	18.6% <0%> (-70.29%)	⬇️
quinn-h3/src/qpack/prefix_string/decode.rs	12.72% <0%> (-67.28%)	⬇️
quinn-h3/src/qpack/vas.rs	97.22% <0%> (-1.84%)	⬇️
quinn-h3/src/qpack/prefix_int.rs	98.46% <0%> (+0.04%)	⬆️
quinn-proto/src/crypto/ring.rs	93.29% <0%> (+0.07%)	⬆️
quinn-h3/src/qpack/encoder.rs	92.91% <0%> (+0.1%)	⬆️
quinn-proto/src/connection.rs	78.6% <0%> (+0.38%)	⬆️
quinn-h3/src/proto/frame.rs	79.03% <0%> (+0.81%)	⬆️

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[Ralith]

This is a beautiful solution, great work! I was imagining much more complex approaches.

[djc]

This is a beautiful solution, great work! I was imagining much more complex approaches.

Me too! I was quite happy that I came up with this little thing last night, although I lost some sleep over it. 😄

[Ralith]

Great stuff!