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.

[huonw]

Nice. I think this looks good, but it's at the limit of my understanding.

Playing back, it looks like the PR has a few different moving parts:

1. setup the PyGeneratorResponseNativeCall type to store a Rust future in a Python object
2. support extracting and awaiting that future in Task::generate (plus GeneratorResponse::NativeCall and generator_send etc.)
3. change how the intrinsics are defined and registered in Rust, which includes removing the Intrinsic and Intrinsics types and the Rule::Intrinsic handling
4. rewrite all the intrinsics to:
   • query the thread-local task context since they no longer get it as an arg
   • wrap their internal future in PyGeneratorResponseNativeCall (which requires a few other adjustments, due to type inference, etc.?)
5. define some normal @rules as wrappers

Then, because we've fully switched over in the definitions, I think this code is being actively exercised by the full test suite. (I'm raising this because: if this was instead a "preparation" that wasn't being used, we might discover a whole bunch of issues when we start using them. But, because they're being used directly by the old test-suite, that's a strong indication that things fit together and there's no behaviour change.)

Have I got that correct (or at least correct enough)?

Approved assuming I have.

[stuhood]

That is all exactly right. Thank you @huonw ! ... I apologize for such a skimpy PR description.