[ refactor ] Change runners to be runnable in pure or purified context

[buzden]

The third solution of a problem discussed in #36

[buzden]

Oh, now reading through the comments in #36, I see that you've changed the style of signatures. In this module I used the style that was already in that module. I think, changing the style should be separate from the proposed change for the future traceability

[stefan-hoeck]

I'm afraid I don't understand how and why this solves the issue discussed in #36. Please add documentation / explanations both to the code and to this PR.

[buzden]

I'm afraid I don't understand how and why this solves the issue discussed in #36. Please add documentation / explanations both to the code and to this PR.

Newly added interfaces now allow to run hedgehog in a pure environment with an ability to analyse the printed output of its checks. I added a little bit of documentation to the interfaces themselves, but more importantly, I added a testing facility which I talked about in the third variant in #36. It tests behaviour of hedgehog itself (including shrinking) both when testing succeeds and fails, an all that is inside hedgehog properties. You can look at the last commit that adds those.

[buzden]

@stefan-hoeck Are my explanations and added tests understandable enough?

[stefan-hoeck]

Thank you very much for this. I'm going to merge as it is. Please rebase #36 if necessary. Afterwards, I'll have another look at that one.