Show example of model-based FsCheck test, to replace tedious Deque tests

I believe the model-based test replaces a lot of the manual tests for Deque. (possibly with a small amount of extra work, like calling head, tail and the active pattern in the check) 

So it is more or less a rough cut, but wanted to share in the hope others can understand and copy the pattern for other data structures, and to test the waters. I intend to work a bit more on this in the future, if you folks like the approach.

In the process I found and fixed a bug in FsCheck, so the update is needed for the test to run correctly.