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 :white_check_mark:

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

```diff @@ 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 ```