jni 0.21 migration

[cpu]

Presently this crate uses jni-rs v0.19 as an optional dependency for doc generation, and a required dependency for the Android OS target.

The latest release stream, 0.21.x, "makes extensive breaking changes in order to improve safety. Most projects that use this library will need to be changed accordingly.". The upstream project offers a migration guide to help with this process.

Given the advertised safety improvements it's likely worthwhile to consider the steps required for migration.

[complexspaces]

Thanks for taking a glance at this! The new safety improvements look very welcome. Our uses are pretty simple so I'm hoping our upgrade path is pretty smooth.

[akonradi-signal]

Is this fixed by #151?

[djc]

Yes.