Iterate over interval boundaries

I think it would make sense to support the iteration interface for `Interval` to return the `left` and `right` boundaries. This would allow more compact destructuring, as in
```julia
lo, hi = 2 .. 4
```
This is nice for example when you have a method which accepts an `Interval`, but really only works with the boundaries. Then you can direct destruct it in the method signature without using property destructuring. The latter one is more verbose and potentially uses private API (not sure whether the field names are considered public API)

The iteration would deliberately not return elements inside the interval. First of all, because we are talking about intervals of a continuous set in general, and secondly, because that's what `Range`s are for.

Start by reading the existing iteration behavior for `Interval` and the package's related tests. Add coverage for destructuring `lo, hi = 2 .. 4`, ensuring iteration yields the `left` and `right` boundaries rather than values inside the interval. Done means boundary destructuring works while preserving the interval's continuous-set semantics.

Written by the indexing model from the issue text.