Nullable and modernization part 2

[erikmav]

Enable nullable annotations for the repo, then update a few files with nullable, convert .NET 1 data structures to strongly typed collections, and perform other modernizations. Plus #region removal per a PR comment elsewhere.

[erikmav]

@FreeAndNil , @fluffynuts - I started a widespread nullable and modernization wave in the code after #119 / #122 , part of which is presented here. If you're willing to accept a series of these, I'm aiming to churn through all the files in the repo to do the same cleanup and optimization. Removing .NET 1.0 ArrayLists and Hashtables in favor of regular and concurrent generics will cut some CPU overhead out of all related code paths.

[FreeAndNil]

Thank you very much for your work.
I've added some remarks and requested some tests (where they are currently missing).
We need to have those methods covered before making changes.

[FreeAndNil]

@erikmav Thank you very much (especially for adding the tests).
After adding the one missing test (RemoveAppender(IAppender) we are ready to merge.

[FreeAndNil]

@erikmav @fluffynuts I've created issue #124 for those improvements.

[FreeAndNil]

@erikmav did you see my comment regarding the missing test?

[erikmav]

Misunderstood the comment. Working on it.

[FreeAndNil]

Thanks a lot.