grouping_map: Allow to create GroupingMap with a custom BuildHasher

[vadorovsky]

Currently, GroupingMap can create a HashMap with the default RandomState.

For some kind of keys, using different BuildHasher implementation can result in better performance. Add a possibility to create a GroupingMap with a custom hasher builder, which then produces a HashMap with it.

[phimuemue]

See also #901 and #906

[phimuemue]

Thanks for this.

Closing this for now, because - if we do this - we won't restrict ourselves to custom BuildHashers, but have entirely different containers.

[jswrenn]

We've been blocked for ages on the problem of abstracting over container type. Abstracting over hasher is a much simpler problem and, with type defaults, perhaps even a non-breaking change. I think we should consider this PR.

[phimuemue]

Re-opening as per @jswrenn's comment.

[phimuemue]

Ok, this looks all reasonable to me.

The only nit I have is that I'd ideally like to have less builder functions. Maybe all ::new-functions could call the respective ::with_hasher-functions, and we could get rid of grouping_map::new?

@jswrenn I'm no expert in the not-a-breaking-change business. I guess this one could be ok, because we "only" add a S: BuildHasher here and there, but existing callers may be unaffected. Could you offer a second look at this, and if you, too, say, this is fine, I'd merge it.

[phimuemue]

It's a nit, but please forward this to with_hasher(iter, RandomState::new()).

[codecov]

Codecov Report

❌ Patch coverage is 46.15385% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.35%. Comparing base (6814180) to head (9dc78fd).
⚠️ Report is 169 commits behind head on master.

Files with missing lines	Patch %	Lines
src/lib.rs	8.69%	21 Missing ⚠️
src/grouping_map.rs	75.86%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1057      +/-   ##
==========================================
- Coverage   94.38%   93.35%   -1.03%     
==========================================
  Files          48       50       +2     
  Lines        6665     6355     -310     
==========================================
- Hits         6291     5933     -358     
- Misses        374      422      +48     

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