Added random function

[mrkbac]

This PR add a random() function.

Example output:

>>> 10 * random() // floor

  floor(10 × random())

    = 6 

Let me know if I should change anything!

[sharkdp]

Thank you!

[Bzero]

I am just a minute too late with my comment but I think it might be worth to think about if the rand() function should take min and max arguments.

For many applications one will need to scale the default [0, 1) interval and to me rand(a, b) looks more convenient than rand() * (b - a) + a.

[triallax]

We don't have optional parameters at the moment so we might want to leave it until that's implemented.

[sharkdp]

@Bzero Thank you for your comment. I had similar thoughts before merging. But I was hoping that we could now implement a function like rand(a, b) or randint(from, to) using that one source of randomness random(). If that turns out to be tricky (e.g. because of subtle biases) , let's add a more FFI functions or rethink this one.

[Bzero]

Yes that sounds reasonable, it would probably make sense to implement these features in a module in numbat itself.

[sharkdp]

to implement these features in a module in numbat itself

Yes, absolutely. That's what I meant. Should have mentioned it. In general, I try to keep the FFI as small as possible. And everything we implement in Numbat itself is a good test for the language.