digest: Add TryFrom<&str + String>

[cgwalters]

Originally I only implemented FromStr but having TryFrom for both &str and String too seems also idiomatic. Notably it's done by Reference, also in this crate.

That said there was a lot of debate in e.g.
https://users.rust-lang.org/t/is-there-actually-a-semantic-difference-between-fromstr-and-tryfrom-str/92765/15

But...these things are strings more than they're not, so arguably indeed TryFrom<string type> makes sense.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.71%. Comparing base (57570dd) to head (6f7eefb).
Report is 37 commits behind head on main.

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Additional details and impacted files

@@             Coverage Diff             @@
##             main     #222       +/-   ##
===========================================
+ Coverage   26.61%   70.71%   +44.10%     
===========================================
  Files          24       26        +2     
  Lines        2788     3159      +371     
  Branches     1516     1678      +162     
===========================================
+ Hits          742     2234     +1492     
+ Misses        948      925       -23     
+ Partials     1098        0     -1098