Parallelize STREAM

[str4d]

Part of #271.

[codecov]

Codecov Report

❌ Patch coverage is 63.96396% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 34.55%. Comparing base (456ce70) to head (4f72ab0).
⚠️ Report is 571 commits behind head on main.

Files with missing lines	Patch %	Lines
age/src/primitives/stream.rs	63.96%	40 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #148      +/-   ##
==========================================
+ Coverage   33.56%   34.55%   +0.99%     
==========================================
  Files          34       34              
  Lines        3081     3102      +21     
==========================================
+ Hits         1034     1072      +38     
+ Misses       2047     2030      -17     

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[paulmillr]

Would be great to have this merged! Filippo doesn't seem to work on age much, so I think it's fine to make rage different in some regards.

[tarcieri]

Would love to have something like this upstream in the aead crate 😉

[str4d]

The current issue with this PR is that it provides way too little performance improvement for the CPU cost it incurs. I still need to dig into this and figure out why.