F sharp d us

As discussed on https://jabbr.net/#/rooms/DDD-CQRS-ES today

Notes:
1. 3rd commit actually compiles (prev 2 can be fixed if you prefer them)
2. Getting stuff into the DU is possible, but more interesting that getting stuff into a normal record e.g. 
    - Json.net supports it but there are reasons not to use it OOTB, see what FunDomain does) 
    - protobuf is also messy, see recent SO posts by me if that matters
3. I think Projac should not do stuff with `FSharpOption` directly - v few libs to this. as demonstrated in the final commit, small one liner glue things in the F# make it work just fine

It might be a good idea to put a `.fs` file with helpers and an example in a nuget that references Projac and let that be the F# support?