Allow generics <TValue, TError> on ValueOrError type

[Odonno]

Linked to #48

It was relatively easy to make the change but I got mad with all these generics everywhere. :) Maybe there is a better way, for now I followed the easy one.

Also, I have 16 failed tests but not directly related to ValueOrError type. Not sure if it is my machine or if the tests are really broken.

[Odonno]

Ok. Seems like 3 unit tests are broken due to JSON conversion. I will change that.

[Odonno]

@DavidArno Well, I made the change on JSON serialization library but I needed to use reflexion methods only available on .NET Standard 2.0. So, it's a breaking change.

And I also need to add more unit tests.

[Odonno]

Still pending.

[DavidArno]

It is indeed. I've been thrashing around with v4 of Succinct for months now. I have multiple branches, with multiple experiments with read-only structs and nullable reference types enabled, all in search of something I'm happy with.

With C# 8 due in the next month, I've created yet another branch and have started the whole process again. So v4 is planned for late September/early October when C# 8 goes live.

[Odonno]

@DavidArno Good to know! Thank you for your dedicated work.