Expose dryoc::error::Error & implement std's Error trait

[hedgehog1029]

This PR exposes dryoc::error::Error as dryoc::Error so that it may be used in external error handling code, and implements std::error::Error.

[codecov[bot]]

Codecov Report

Merging #3 (6467bde) into main (52ad1c2) will increase coverage by 1.26%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main       #3      +/-   ##
==========================================
+ Coverage   81.60%   82.86%   +1.26%     
==========================================
  Files          43       43              
  Lines        4310     4277      -33     
==========================================
+ Hits         3517     3544      +27     
+ Misses        793      733      -60     

Impacted Files	Coverage Δ
src/error.rs	14.28% <0.00%> (-19.05%)	:arrow_down:
src/lib.rs	100.00% <ø> (ø)
tests/integration_tests.rs	100.00% <0.00%> (ø)
src/protected.rs	56.12% <0.00%> (+0.40%)	:arrow_up:
src/dryocbox.rs	82.90% <0.00%> (+0.42%)	:arrow_up:
src/argon2.rs	94.67% <0.00%> (+0.58%)	:arrow_up:
src/poly1305.rs	98.93% <0.00%> (+1.04%)	:arrow_up:
src/keypair.rs	58.22% <0.00%> (+1.43%)	:arrow_up:
src/dryocstream.rs	88.69% <0.00%> (+1.51%)	:arrow_up:
src/pwhash.rs	47.82% <0.00%> (+1.58%)	:arrow_up:
... and 3 more

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[brndnmtthws]

Looks great, thanks for the PR. I think we can skip the unit tests for printing errors.

[hedgehog1029]

just noticed that I accidentally removed the #![warn(missing_docs)] line in lib.rs, apologies!