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

Attention: Patch coverage is 40.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 93.80%. Comparing base (6814180) to head (84b2e56).
Report is 149 commits behind head on master.

Files with missing lines	Patch %	Lines
src/all_equal_value_err.rs	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1032      +/-   ##
==========================================
- Coverage   94.38%   93.80%   -0.59%     
==========================================
  Files          48       51       +3     
  Lines        6665     6202     -463     
==========================================
- Hits         6291     5818     -473     
- Misses        374      384      +10     

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[phimuemue]

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

[phimuemue]

Rebased against our main branch.