Use FsCheck

Hello,

I wanted to start playing with [FsCheck](https://github.com/fscheck/FsCheck) for a while. i will let FsCheck introduce itself:

> FsCheck is a tool for testing .NET programs automatically. The programmer provides a specification of the program, in the form of properties which functions, methods or objects should satisfy, and FsCheck then tests that the properties hold in a large number of randomly generated cases. While writing the properties, you are actually writing a testable specification of your program. Specifications are expressed in F#, C# or VB, using combinators defined in the FsCheck library. FsCheck provides combinators to define properties, observe the distribution of test data, and define test data generators. When a property fails, FsCheck automatically displays a minimal counter example.

I will start a branch about it next Thursday.

Thank you @thinkbeforecoding for mentioning it and /u/Frozen_Turtle to mentioning again. Lets do it!

The issue names no files, tests, or entry points. Start by reviewing the repository structure and current test setup, then determine where FsCheck would fit and which properties should be introduced; done should mean a defined FsCheck integration with passing property-based tests.

Written by the indexing model from the issue text.