Add workaround for JNA bug when passing small structs by value.

[rfk]

The current version of JNA appears to have a bug when passing small structs
as arguments by value, on arm64 platforms, when the function call has too
many arguments to pass the struct in registers. It turns out that our generated
code does this a lot thanks to the way we pass around RustBuffer structs.

This commit adds a temporary workaround for the issue, by padding our structs
to be larger than 16 bytes, which causes them to be passed as pointers rather
than by value.

See mozilla/application-services#3646 for investigation
of the JNA issue, and #334 to track
the work of removing this workaround once a fix is released in upstream JNA.

[rfk]

@travis79 I made a local build of the glean+nimbus sample app using this patched version of uniffi, and AFAICT it resolves the arm64 bug I was seeing with invalid memory reads. Any chance you could try this out on your device that was also experiencing a crasher, and see if it works for you? You'd need to:

• Configure your nimbus-sdk rust project to use this version of uniffi, e.g. using a git commit dependency.
• Install this version as uniffi-bindgen in your path (so it can be found by the gradle script to generate Kotlin bindings.

I'm confident enough that this resolves a real issue that I think we should proceed with merging and cutting a new uniffi release that includes it, but it would be worth getting confirmation from your device as well for completeness.

[rfk]

Here's the concrete repro of the bug, for context: https://github.com/rfk/arm64-jna-structbug

[rfk]

Also for context, the upstream bug report: java-native-access/jna#1259

[eoger]

Oh man this is a pretty bad bug, good job investigating it!

[travis79]

@travis79 I made a local build of the glean+nimbus sample app using this patched version of uniffi, and AFAICT it resolves the arm64 bug I was seeing with invalid memory reads. Any chance you could try this out on your device that was also experiencing a crasher, and see if it works for you? You'd need to:

* Configure your nimbus-sdk rust project to use this version of uniffi, e.g. using a [git commit dependency](https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#specifying-dependencies-from-git-repositories).

* Install this version as `uniffi-bindgen` in your path (so it can be found by the gradle script to generate Kotlin bindings.

I'm confident enough that this resolves a real issue that I think we should proceed with merging and cutting a new uniffi release that includes it, but it would be worth getting confirmation from your device as well for completeness.

This is a little non-trivial to test locally because the application I was testing this in was depending on the nimbus in the appservices megazord. Let me see if I can build and publish the megazord locally. I trust your judgement that this resolves the issue if we want to go ahead and merge this, it might make it a little easier for me to confirm if it percolated down to the prototype megazord with nimbus in it.

[rfk]

@travis79 FYI the 0.4.0 release is out with this fix included.