Canonicalize all_equal_value's error

[phimuemue]

Similar to exactly_one_error's error type, all_equal_value's error type now implements std::error::Error. (I saw the discrepancy because exactly_one_error works with anyhow, whereas all_equal_value does not.)

Since I had to introduce a new type anyways, I converted from (Item, Item) to [Item; 2]. I suggest we generally lean towards arrays instead of tuples if the components have the same type.

[codecov]

Codecov Report

❌ Patch coverage is 46.15385% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.92%. Comparing base (6814180) to head (54edb63).
⚠️ Report is 202 commits behind head on master.

Files with missing lines	Patch %	Lines
src/all_equal_value_err.rs	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1032      +/-   ##
==========================================
- Coverage   94.38%   93.92%   -0.46%     
==========================================
  Files          48       52       +4     
  Lines        6665     6638      -27     
==========================================
- Hits         6291     6235      -56     
- Misses        374      403      +29     

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[phimuemue]

Checks fail because Display implementation is not covered, similar to the existing ExactlyOneError.

[phimuemue]

Rebased against our main branch.

[jswrenn]

@phimuemue Can you rebase + add an entry to the changelog? Since we're cutting a breaking release anyways, I'd like to include this.

[phimuemue]

@phimuemue Can you rebase + add an entry to the changelog? Since we're cutting a breaking release anyways, I'd like to include this.

I tried via the web UI, so I'm not sure if it worked. If not, I'll try to get back to this this evening.

[jswrenn]

Thanks! Feel free to hit the merge button once the CHANGELOG has an entry for this. :-)

Are you at RustWeek by any chance?