Abstract state machine testing

The F# version should also support State Machine Testing similar to the Haskell version.

See https://github.com/hedgehogqa/haskell-hedgehog/pull/89, and at least up to https://github.com/hedgehogqa/haskell-hedgehog/pull/96.

To have a solid foundation to build on, we first need to have `Tree` and `Gen` data types and combinators in line with the Haskell version, as per https://github.com/hedgehogqa/fsharp-hedgehog/issues/111 and https://github.com/hedgehogqa/fsharp-hedgehog/issues/89.