feat(bindgen): add support for stream reads with count > 1

[andreiltd]

This adds readMany(count) support to generated p3 streams so host-side byte consumers can perform Canonical ABI stream reads instead of pulling one byte at a time. It uses the transferred count from the packed stream result when reading host buffers.

I believe that the current implementation is correct, it's just that iteratrd on single bytes is very inefficient. I originally came across this when tried to run p3_cli_hello_stdout test from wasmtime:

    async fn run() -> Result<(), ()> {
        let (mut tx, rx) = wit_stream::new();
        futures::join!(
            async {
                wasi::cli::stdout::write_via_stream(rx).await.unwrap();
            },
            async {
                tx.write(b"hello, world\n".to_vec()).await;
                drop(tx);
            },
        );
        Ok(())
    }

The write function is a bit surprising in the sense that it will not loop until the data is sent over, but instead it will try to send the data at most once. In jco we will receive a single byte and report transferred = 1, which is correct behavior. The looping function is write_all which with our current implementation will indeed print the whole string.

I need to have a look if tx.write reports warning that we are dropping result that must be used, which would make the debugging much easier.

[vados-cosmonic]

Hey @andreiltd should I be reviewing this just yet? You just rebased I think but should I wait for the other PRs to land in main?

[andreiltd]

Hey @vados-cosmonic yeah, not yet, this depends (includes): #1445

but it looks like 1445 is failing so I will have to figure out that first and then I will rebase this one.

[andreiltd]

It's ready now.