Improve Unique[By]/Duplicates[By] debug messages

[FHTMitchell]

Updates the Debug impls of Duplicates[By] and Unique[By] to be more descriptive. Hides the implementation detail that seen is a HashMap under the hood for Unique[By] and removes the meta redirect in Duplicates[By].

Open questions:

1. Unique doesn't actually get any benefit from having an inner: UniqueBy field since it has it's own next impl. Should we just lift those fields into the struct?
2. I renamed used to seen which feels like a more correct name, but I can revert.
3. I made the *By structs use finish_non_exhaustive which adds a .. to the representation. That's not something anything using the macro gets but useful information I feel. Might be worth adding a .. option to the macro?
4. Should the values in seen in Duplicates[By] become a 2-valued enum (not a bool) to make it clearer what's going on? Probably not worth it...

 // Before

  Unique { iter: UniqueBy { iter: Iter(["b", "c"]), used: {"a": ()} } }
  UniqueBy { iter: Iter(["b", "c"]), used: {1: ()} }
  DuplicatesBy { iter: Iter([3]), meta.used: {1: false, 2: true} }
  DuplicatesBy { iter: Iter([3]), meta.used: {0: true, 1: false} }

// After

  Unique { iter: Iter(["b", "c"]), seen: {"a"} }
  UniqueBy { iter: Iter(["b", "c"]), seen: {1}, .. }
  Duplicates   { iter: Iter([3]), seen: {1: false, 2: true}, pending: 1 }
  DuplicatesBy { iter: Iter([3]), seen: {0: true, 1: false}, pending: 1, .. }

[codecov]

Codecov Report

❌ Patch coverage is 42.00000% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.57%. Comparing base (6814180) to head (19e625d).
⚠️ Report is 211 commits behind head on master.

Files with missing lines	Patch %	Lines
src/unique_impl.rs	54.54%	15 Missing ⚠️
src/duplicates_impl.rs	17.64%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1121      +/-   ##
==========================================
- Coverage   94.38%   93.57%   -0.82%     
==========================================
  Files          48       52       +4     
  Lines        6665     6708      +43     
==========================================
- Hits         6291     6277      -14     
- Misses        374      431      +57     

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