[v6] Remove all dedicated C# interop features from the core lib

[kMutagene]

Since we now aim to provide (hopefully) auto-generated native C# bindings, features such as the [<Optional>] and [<DefaultParameter>] attributes for function arguments and the fluid extension in the F# API can be removed and the interop can be handled internally.

[kMutagene]

This has the added benefit that C# bindings will not compile anymore when new arguments in the core lib are not included in the bindings.