Slightly refactor code to make it look fresh

[zvirja]

In this PR I did a bit of polishing to make our code look nicer, cleaner and easier to read and maintain. Here is an approximate list of changes:

• Consume the latest language features to make code look fresh and more concise
• Convert some of the methods to lambdas. I made case-by-case decisions to ensure code looks better than before.
• Remove ArgumentNullException checks from the internal parts of the code (i.e. mostly NSubstitute.Core), as now we have nullability in place to verify code is right. Code was not covered entirely anyway, so should not be an issue. If we have advanced clients who are using advanced API and push null values - well, either enable nullability or just be a nice person then 😊
• Arrange missing visibility modifiers for consistency.
• Did a bit of local refactorings to make code look nicer.
• [Done via a separate commit] Split SubstituteExtensions into multiple partial classes, so that it's easier to navigate and maintain. Before it was just one huge junk with everything there.
• [Done via a separate commit] Add a few R# configurations, as today we use either R# or Rider widely, so I wanted our code to look consistent.

There might other changes apart from the mentioned. Please review them if you don't like particular changes, so we could discuss those. Also I could completely revert/descope the individual commits if you would like to review/discuss those separately!

Thank you for your time!

[dtchepak]

LGTM! 👍 🎉