Adding ADR-0009

[bendk]

Use handle map handles to pass objects across the FFI,

[bendk]

I'll state my preference up-front: I think we should be using handles.

I don't like 1 because it's too hard to debug errors. Whenever I want to add new functionality that involves passing an object, I almost always mess it up at least once and currently this results in an impossible to understand stack trace or a crash with no message at all. When I was trying to switch things over to using a handle map in #1808, I definitely had bugs like this, but getting the nice error message was much nicer.

I don't like 4 because I don't want to add the dependency for all users.

I slightly prefer 3 over 2, but I'd be fine with either of those.

[bendk]

I think the changes in #1823 and #1826 are good regardless of this decision, so the ADR doesn't mention them. Please speak up if you think some of those should be part of the ADR, or another one.

This ADR might guide how some of that code looks, for example naming the variant FfiType::Handle makes more sense if we choose option 2 or 3.

[badboy]

Rendered

[bendk]

Just pushed some small changes to the text and added a document that tries to describe how the handlemaps would work in detail: https://github.com/mozilla/uniffi-rs/blob/6307aa0561312472557c8770da1042a7d74a24ad/docs/handles.md