Expose intrinsics as decorated Python functions instead

[stuhood]

Remove intrinsics facility from the engine, and instead expose intrinsics in intrinsics.py via a new native call response for generators. This solves an issue in #19730 that there were no obvious @rule functions to call for intrinsics.

The native call response wraps a Rust future, which we trampoline to execute on the Rust runtime, and then return a value to the generator. It should work generically for any use case where we would like @rule code to await native code, which opens the door to fixing #11329 by making Workspace.write_digest async.