Support validating nullable iterables

[nesk]

The following functions should support nullable iterables:

public operator fun <T> Validatable<Iterable<T>>.iterator(): Iterator<Validatable<T>>

public inline fun <T> Validatable<Iterable<T>>.each(block: Validatable<T>.() -> Unit)

Currently, the following code doesn't work, each isn't callable:

Validator<List<Any>?> {
    each { /* ... */ }
}