Switch to C#8 and use Nullable Reference Types annotations

[akamsteeg]

We should try to switch to C#8 with Nullable Reference Types for this lib:

Tasks:

• [x] Run the unit tests on .NET 4.7.2 to make sure we don't break anything on NETFX;
• [x] Avoid using any C#8 features on the public API. No !is, etc. Leave the argument checking intact. We must guarantee that we don't break things on NETFX. Keep in mind that using some things from the internal API might bubble through to the public API;
• [x] Remove all internal argument checking where applicable (e.g. private methods called by public methods with argument checking). When nullable reference types guarantees a not-null situation at compile time, we don't have to check it at runtime.
• [ ] ValueTask<T> internally, where applicable. This is a hard one. Any TFM above .NET Standard 2.0 is not a good idea right now. We want to support a wide range of TFM's (.NET 4.7.2+, .NET Standard 2.0+) and that precludes the usage of ValueTask<T> currently. (See https://github.com/akamsteeg/AtleX.HaveIBeenPwned/issues/31#issuecomment-534633024)

[akamsteeg]

I decided to hold off on using ValueTask/ValueTask<T>. I realize its benefits, but currently we would introduce a new dependency for NETFX and that's not worth the risk I think. Especially when the benefits for NETFX are questionable right now and we have a lot of looooooooong-running things going out of process (calling the HaveIBeenPwned.com API). Is it worth using ValueTask/ValueTask<T> right now? I think it just complicates matters so I'll wait.