Last-In, First-Out enumeration of an event stream

This Pull Request adds a `LifoEvents<T>` class, as described in #83.

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I didn't add facade tests in [DataContractEventsFacadeTests](https://github.com/GreanTech/AtomEventStore/blob/master/AtomEventStore.UnitTests.FSharp/DataContractEventsFacadeTests.fs) and [XmlEventStreamFacadeTests](https://github.com/GreanTech/AtomEventStore/blob/master/AtomEventStore.UnitTests.FSharp/XmlEventStreamFacadeTests.fs) F# modules – if I have to, please let me know.