Tuples: adding With(..) and OfSize(..) helpers

- FdbTuple.Create(123, "abc").With((x, y) => ... ) will have x == 123 and y == "abc"
- tuple.OfSize(3).Foo(...) will verify that tuple.Count == 3, and throw if this is not the case.
- inspired by https://github.com/louthy/language-ext !