[Proposal] Expose read access for BackStack#elements

[friendoye]

I wonder if there is any way to retrieve all elements from BackStack?
It can be useful in the following cases:

1. We want to render all screens from BackStack on top of each other, so after navigating back we will not re-render some screens.
2. Integration with compose-backstack (library for rendering animated transitions between backstacks of screens) would be much simpler, because nowadays it requires list of screens as one of args to its Composable. There are some workarounds, such as using Kotlin Reflection or using state/remember for maintaining backstack yourself, but it would be great to have read access directly to BackStack#elements.

@zsoltk WDYT?

[zsoltk]

I don't see why not. As a short term fix, it will do. Feel free to submit a PR.

Otherwise an abstraction over the back stack is on the way to address a more general case. That should also allow for keeping more than one element alive, among other things.

[friendoye]

Sounds great, will look forward to new abstraction!

[friendoye]

Closed by #37.