Random BigInt between two long long numbers

[wilky-way]

No description provided.

[faheel]

@awilki13 Thanks for the PR, but having a function to generate a BigInt between two long long ints isn't really useful here. What would be useful is a function that generates a random BigInt between two given BigInts (arbitrary-sized) as mentioned in #19.

To be clear, it should two arguments (low and high) whose type can be any combination of a BigInt, a string or long long int (6 overloaded functions in total), which should return a random BigInt that is between low and high.

I'm closing this PR for now, but in case you decide to implement it, feel free to reopen it and add a comment, and I'll review it. Thanks again for contributing.