Implement non-Promise collapsed future<future<T>> lowering in async calls

[vados-cosmonic]

Lowering values of future<future<t>> are more complex than normal because in JS Promise<Promise<T>> is automatically resolved to Promise<T>. To properly represent nested futures and use them, we must:

• Spontaneously create the right number of component-internal future waitable objects
• Ensure that when host writes are injected (i.e. just before a guest read where the host holds the write end), we possibly turn the value into a promise
• Lifting a nested future wraps the value going out into something that behaves like a Promise<Promise<T>> without collapsing
• For the component the future is being lowered into, and make sure that when

As of #1422 there is a partial implementation of nested stream lowering in the Instruction::FutureLower instruction, which produces a "Thenable" that does essentially counting to replicate the idea of a future that resolves nestedly, but this implementation does not yet work and neither does LowerIntrinsic::LowerFlatFuture.

(we should also de-duplicate both lowers but that's a discussion for another day)

An entirely workable solution is also to require that nested futures passed from javascript are of a different form, or to treat futures as AsyncIterators, rather than Promises. This may be preferable to actually trying to coerce Promises to do what we'd like them to do.