Parallelize STREAM

[str4d]

Part of #271.

[codecov]

Codecov Report

Merging #148 (4f72ab0) into main (456ce70) will increase coverage by 0.99%.
The diff coverage is 63.96%.

@@            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     

Impacted Files	Coverage Δ
age/src/primitives/stream.rs	58.13% <63.96%> (-1.35%)	⬇️
age/tests/test_vectors.rs	79.16% <0.00%> (-4.17%)	⬇️
age/src/format.rs	58.02% <0.00%> (-3.23%)	⬇️
age/src/protocol.rs	68.79% <0.00%> (-1.92%)	⬇️
age/src/keys.rs	80.00% <0.00%> (-1.82%)	⬇️
age/src/encrypted.rs	41.30% <0.00%> (-1.71%)	⬇️
age/src/plugin.rs	4.58% <0.00%> (-0.46%)	⬇️
age/src/util.rs	57.14% <0.00%> (ø)
age/src/error.rs	2.23% <0.00%> (ø)
... and 14 more

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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.