isUnmodifiable assertion for Iterator instances

[scordio]

Feature summary

Similarly to #2102, we could have an isUnmodifiable() assertion for Iterator types.

Example

Iterator<Integer> iterator = Collections.unmodifiableList(Arrays.asList(1, 2, 3)).iterator();

assertThat(iterator).isUnmodifiable();

[dehasi]

@scordio if nobody works on this task, I'd like to pick it up. And it seems a bit more challenging than for collections :)

[scordio]

Sure and thanks, @dehasi!

What challenges have you encountered?

[dehasi]

To be short, I can't rely on that iterator.remove() will throw UnsupportedOperationException.

I.e. Collections.EmptyIterator.remove() throws IllegalStateException. Do we treat EmptyIterator as unmodifiable?

But can't rely on IllegalStateException as unmodifiability marker. ArrayList<>().itertator() can also throw IllegalStateException if we haven't called next.

Initially, I wanted to check unmodifiability by just calling iterator.remove(). It's not enough (see to ArrayList example).
Now, I'm thinking of making an additional check like iterator.next(); iterator.remove(). However, it will look a bit weird.