Added SliceStream and SliceListStream

- SliceStream wraps one Slice into a Stream
- SliceListStream wraps multiple slices - sequentially - into a single stream
Added [ImmutableObject] attribute on IFdbTuple and Slice

- This is so that the IDE Designer shows the fields as readonly
Merge branch 'master' of github.com:Doxense/foundationdb-dotnet-client