Support output iterators

[Manishearth]

Split out from #251

There are two things here. Iterators and Iterables. Iterables are the equivalent of IntoIterator, which themselves store no iteration state but can be "iterated over", producing a temporary iterator type. Iterators are the equivalent of Iterator which store iterator state and can get "exhausted" once iterated.

Most language with iteration glue have that glue prioritize iterables, not iterators, though usually there's a standard way to make iterators be iterables as well. For example, in Javascript, iterables have an @@iterator method that produces an iterator, and anything with a next() method that returns an object with done: bool and value fields. The way you make an iterator also an iterable is just to give it a [Symbol.iterator]() {return this;} method.

From an ICU4X standpoint it's more important for us to prioritize iterators; most of the time we have methods that take additional parameters to produce an iterator, so they're not themselves iterable.

Roughly, we should do something like this:

#[diplomat::opaque]
struct ICU4XLineBreaker(..);

impl ICU4XLineBreaker {
    pub fn break(&self, text: &str) -> ICU4XLineBreakIterator {..}
}


struct ICU4XLineBreakIterator<'a>(...);

impl<'a> ICU4XLineBreakIterator<'a> {
    #[diplomat::attr(*, iterator_next)]
    pub fn next(&self) -> Result<SomeType, ()> {
      // ...
    }
}

An actual next() method is not generated (in the long run we can make htis configurable). This would generate an ICU4XLineBreakIterator struct with a properly wrapped next() class that produces the {done, value} enum and additionally has a return this @@iterator method.

We might want to add a supports=iterators name/value Diplomat attribute CFG as well.

For iterables, we can also add a #[diplomat::attr(*, iterable)] type that can be tagged on a single method that has no additional parameters to mark things.

Originally posted by @Manishearth in #251 (comment)