Checklist for the v4 release

List still WIP. Just noting things down as I think of them:

[x] Remove Maybe<T>. See #57
[x] Convert Option<T> and the Union<> types to readonly structs. See #57
[x] Change the way Succinc<T> interacts with null. See #51.
[x] Remove Ignore. See #59
[x] Make ValueOrError and Success types, readonly (currently just structs).
[x] Switch the supported framework to netstandard 2
[x] Add @Odonno's With features (See #44).
[x] Address issue/PR #48 (@Odonno)
[x] Address issue/PR #49 (@gavinSteyn) Care needs taking to ensure it doesn't cause problems with C# 8's pattern matching later (I don't think it will, but want to check).
[x] Address issue/PR #50 (@xlecoustillier)
[x] Address issue/PR #51 (@xlecoustillier).
[x] Make use of the new Enum generic constraint to tidy up the code around the enum parser (#64)
[x] Check docs for the types that have been changed to structs to see if they need updating to mention this
[x] Ensure the way that null equality is now handled by option and unions is documented
[x] Document removal of Ignore (use discards instead)
[x] Check whether the enum parser docs mention anything about throwing exceptions if passed a non-enum and remove if they do
[x] document the new features (this point needs expanding to cover each of them

DavidArno / SuccincT