Switch to a HashSet<T> as backing for SafeEnumerableList

[halgari]

What does the pull request do?

Switches SafeEnumerableList to SafeEnumerableSet, in order to offer better performance on the .Add and .Remove methods

What is the current behavior?

As the Classes class adds and removes listeners it stores them in a SafeEnumerableList. When listeners are removed they are removed from this list. However, since the backing store for this collection is a List<T> the .Remove method becomes a O(n) scan on the items. In our application we were seeing significant pauses in the application when the number of listeners grew rather large. In our case one of the lists had about 700 listeners, so the pauses became quite large.

In addition, if it is assumed that listeners that are added recently (like those deep in a tree) will be removed first, we can see another problem: the items we remove will most likely be at the end of the collection.

What is the updated/expected behavior with this PR?

The backing store is now a HashSet. None of the public APIs have changed in behavior, although there may be slight differences in performance, similar to those expected from switching from a list to a hashset.

How was the solution implemented (if it's not obvious)?

Pretty much the same as the previous solution except for the following changes:

• We can't do a _list[idx] inside the Enumerable class as HashSets don't support offset indexing. Instead when we create the enumerable we wrap the specialized .NET HashSet enumerator, and add our functionality to this method. This also means that we no longer need a few of the fields on Enumerable
• We have to clone the HashSet if it is modified during enumeration. This is supported via new HashSet(_set) which has an optimized fastpath in the .NET Constructor. It is true that cloning a hashset is slower than cloning an array, but it is assumed that the "modified while enumerating" usecases are rare.

Checklist

• Added unit tests (if possible)?
  • Existing tests were updated
• Added XML documentation to any related classes?
  • No classes added
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation
  • Internal class, not required

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0051095-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[halgari]

@cla-avalonia agree

[MrJul]

LGTM, this change makes perfect sense.
Thank you for your contribution!

[kekekeks]

Note that this PR isn't safe to backport since it technically changes the enumeration order, which can have some effects on the way styles get attached/detached.