Fix non-iteratrable input of yieldEach

[Yi2255]

Logic:

1. Try to use existing variables and avoid creating new ones proactively, as this can lead to uninteresting and meaningless code.
2. If the variable passed to the current CodeGenerator is iterable, use it directly.
3. If it is not iterable, randomly generate an array composed of variables from the context, which, in the worst case, will still be empty, ensuring that what follows yieldEach is an iterable object, thus eliminating the need for additional guard checks.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[saelo]

Thanks!

[saelo]

I think here it would make sense to split this into two generators:

    CodeGenerator("YieldGenerator", inContext: .generatorFunction, inputs: .one) { b, val in
        assert(b.context.contains(.generatorFunction))
        if probability(0.9) {
            b.yield(val)
        } else {
            b.yield()
        }
    },

    CodeGenerator("YieldEachGenerator", inContext: .generatorFunction, inputs: .required(.iterable)) { b, val in
        assert(b.context.contains(.generatorFunction))
        b.yieldEach(val)
    },

That way it's easy to guarantee that we get a .iterable.

[saelo]

Here and in the AsyncGeneratorFunctionGenerator, I would recommend doing this differently. For one, I think we would usually prefer to yield something that we create in this function (i.e. in the buildRecursive() call), but that's not possible if the it comes from the outside. I would probably recommend just doing something like this:

} else {
    let randomVariables = b.randomVariables(Int.random(in: 1...5))
    let array = b.createArray(with: randomVariables)
    b.yieldEach(array)
}

This will then cause us to always yield a plain array, but we have the other CodeGenerator below for yielding other stuff. Also the InputMutator will quickly shuffle this around to yield from other things.