Added test stubs for remaining exercises of chapter 5.
Exercises 6.{1,2,3,4}: Implemented methods for generating random numbers by a RNG.
Added some documentation for type Rand[+A].
Exercise 6.{5,6}: Implemented functions in terms of map.

These implementations don't explicitly mention or pass along the RNG value. The
map combinator and the introduced (function) type Rand[+A] helps to implement
these functions in a succinct and elegant way.