Feature: n-dimensional product over the same iterator

So that code like this:

```rust
iproduct!(0..4, 0..4, 0..4)
```

becomes the more succinct:

```rust
iproduct!(0..4; 3)
```

This would mandate vectors instead of tuples for the items.
It would help for when the dimension isn't known until runtime:

```rust
fn truth_table(dim: usize) → Box<Iterator<Item = Vec<bool>>> {
    Box::new(iproduct!([false, true].iter(); dim))
}
```