Updating UniFFI to 0.29.5

[bendk]

This brings in mozilla/uniffi-rs#2733, which I'm hoping will reduce the crash frequency for
https://bugzilla.mozilla.org/show_bug.cgi?id=1992149. This is pretty speculative, but it seems worth a shot.

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[rvandermeulen]

Tests are green and Mark reviewed the UniFFI change already in the upstream repo. Let's get this landed and into Nightly builds ASAP.