Fix for multiple crates defining async functions
A single global callback won't work if there are multiple foreign
modules.
In `main` we handle this by inputting a callback in `poll()`.
In `release-v0.25.x` we can't change `UNIFFI_CONTRACT_VERSION`, so
instead we keep the `rust_future_continuation_callback_set` scaffolding
function, then have the scaffolding code pass the callback to
`rustfutures.rs`.
Added some tests for this in `fixtures/ext-types/proc-macro-lib`.
Things worked on Swift and Python for me because that code was based on
leaking a pointer and it happened to work if the pointer came from
another module. It just halted on Kotlin though and I think the same
would happen if I wasn't on CPython and the alternate PointerMangager
was used.
