Implement user sessions

[ocramz]

(as already mentioned in the urlshortener example program)

With cookies #69 and a proper exception system in place #310 it should be possible (and fairly easy) to implement a "session" interface like Flask and similar have:

https://testdriven.io/blog/flask-sessions/

See also #59

[ocramz]

code from here could be recycled for this purpose : https://hackage.haskell.org/package/scotty-session-0.0.5/docs/Web-Scotty-Session.html#t:ScottySM

but using scotty internal cookie functions

[k0001]

I just published wai-cryptocookie, a small wai library that maybe could be used to implement this.

https://hackage.haskell.org/package/wai-cryptocookie/docs/Wai-CryptoCookie.html

The library doesn't depend on anything other than what scotty already depends on.