Wrap the array representation of opaque types in a #[repr(C)] struct

[GKFX]

Fixes #2845. Wrapping the generated array in a struct ensures that it is passed in an FFI-safe way. This does assume that the calling convention for structs is only dependent on size and alignment, but that seems like an improvement on using a type whose calling convention is not considered stable at all. Padding bytes (etc.) have been excluded from this treatment.

[GKFX]

@stevecapperarm does this resolve your issue in #2845?

r? @emilio @pvdrz

[rustbot]

Error: The feature assign is not enabled in this repository. To enable it add its section in the triagebot.toml in the root of the repository.

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #t-infra on Zulip.