Use HashSet from BCL instead of custom implementation.

[Giorgi]

Use generic System.Collections.Generic.HashSet from BCL instead of implementing it manually.

[martinwoodward]

Thanks for the PR @Giorgi! It may take us a while to get to this one (see the Historical Note section in Scott's blog) because we're heads down focusing on the Blogger API fix (see #63 and #5) and then some other crashes and high priority bugs we're seeing such as #26 etc. There sure is a lot of code improvements that could be made though once we have a stable release that is working for the majority of folks so thanks for taking a look.

[Giorgi]

@martinwoodward I agree, shipping feature is more important. Do you think we need a new tag for PR-s that are refactoring and/or getting rid of legacy code so that we can easily track such issues?

Having a readable code is important too because it makes contributing easier. Most of the changes in this PR are just adding new using statement and a generic type parameter.

[kathweaver]

From @ScottIsAFool on another one: We have moved to the dotnetfoundation's AppVeyor account and ScottH has closed his account which is what we used to be using. If @vhanla were to make an arbitrary commit to this branch and push it, it would force a rebuild on appveyor.

I'll test once it's updated.

[RobDolinMS]

@Giorgi BIG THANK YOU for submitting this PR.
As we moved to a new AppVeyor account, would you mind making a minor update (to trigger a new build)
THANK YOU again for the PR and sorry for the long-delayed review.

[ScottIsAFool]

Hi @Giorgi, it looks like there are conflicts at the moment that need to be resolved in order for this pull request to be built, could you please resolve these.