util: use subtle for XORing bytes

[Jorropo]

Fixes #377

[Jorropo]

Revisit when updating to go1.21

[codecov]

Codecov Report

Merging #379 (ec910c3) into main (56861d6) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #379      +/-   ##
==========================================
+ Coverage   66.41%   66.48%   +0.06%     
==========================================
  Files         207      207              
  Lines       24692    24692              
==========================================
+ Hits        16400    16416      +16     
+ Misses       6866     6852      -14     
+ Partials     1426     1424       -2     

Files	Coverage Δ
util/util.go	33.33% <100.00%> (ø)

... and 15 files with indirect coverage changes