Add OrderedDictionary

[stephentoub]

Closes #24826

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[kronic]

@stephentoub @terrajobst
Maybe it's worth adding more methods

public void Move(int oldIndex, int newIndex);
public void Stort(IComparer<TValue> comparer);

[eiriktsarpalis]

Adding some kind of sorting functionality would be useful, but ideally that should be solved for IList in general, cf. #76375

[stephentoub]

@eiriktsarpalis, please take another look when you get a chance. I redid the guts based on the feedback to prioritize enumeration. It's based on a combination of approaches taken by Dictionary<> and by the corefxlab OrderedDictionary<>.

[stephentoub]

/ba-g all failures are known

[AndreyAkinshin]

@stephentoub Thank you very much for doing this! It's fantastic that we can finally use generic OrderedDirectiory out of the box without using 3rd-party NuGet packages or implementing it from scratch.

[eiriktsarpalis]

Is there a chance we could perf improvement if the shifting of the entries array used block copying?

[stephentoub]

Possibly. But we still need to walk the entries individually to fix up the chains. We can more easily experiment with it once it's in a daily.