Optimize new ToHashSet api.

[jamesqo]

dotnet/corefx#13726 added Enumerable.ToHashSet, which is currently equivalent to writing new HashSet<TSource>(items). We may want to look into optimizing it by adding it as another method on IIListProvider, and reducing some virtual method calls. Opening this as a tracking issue for that.

cc @JonHanna

[JonHanna]

Most optimisations available from IIListProvider are available due to being able to:

1. Predict the size of the collection produced from the size of the source.
2. Pass that information to the collection creation.

In the case of HashSet, knowledge of the source size only gives a maximum, because the size will depend on the number of distinct elements (and that in turn on the equality criteria), so a 10,000 element source could end up creating a 3-element set. There also isn't a constructor that takes a capacity argument (for much the same reason).

[jamesqo]

@JonHanna You are correct, most of the time (except for a few fringe cases like Range, Repeat, and Union) we will not be able to predict the size of the end result... however, we can still reduce virtual method calls (i.e. like here) by just looping through the source in Select and Where, and doing the exact same thing as if we had been called MoveNext on.

[JonHanna]

Ah, I get you. That would reduce some calls, all right, and not interfere with the optimisations within HashSet itself, for that matter, AFAICT.

[felipepessoto]

Hi, I'll work on it.
@karelz would you assign me please?

The API isn't approved yet, do you think it worth the time to work on it?

[karelz]

This is no API addition, but perf enhancement. Upfront approval is not necessary.
If you want to focus on impactful issues, check out "up for grabs" + (wishlist or milestone:2.0.0).

[felipepessoto]

Since we have only a few hours to work on it, we'll start with the small issues to learn how the entire process work and motivate people to contribute often
@TiagoSoczek will take this issue

[felipepessoto]

@karelz would you assign @TiagoSoczek?
Thanks

[karelz]

Invite sent ...

[TiagoSoczek]

Invitation accepted @karelz, thanks. I'll work on it.