update future.{write,read} ABIs

[dicej]

This updates the Rust generator to use the new ABI defined in WebAssembly/component-model#524. The C generator will need a similar update as well.

• future.write now accepts its payload value as up to 4 flat parameters before spilling to linear memory.

• future.read takes no payload pointer when it has no payload type

• {stream,future}.close-{readable,writable} have been renamed to {stream,future}.drop-{readable,writable}

I've tested this end-to-end with wasip3-prototyping and will soon post a corresponding PR there.

Finally, rustfmt was complaining about using gen as a field name, so I took this opportunity to address that.

[dicej]

I should note that this doesn't cover all of WebAssembly/component-model#524. Specifically, these items will need work (presumably just runtime test updates in this repo once wasip3-prototyping has been updated):

• There is no "number of elements written" packed in the high 28 bits of the future.{read,write} results.
• On successful copy, future.{read,write} return COMPLETED instead of CLOSED (which, as noted above, was renamed to DROPPED, making it especially "wrong" as the result code).

[dicej]

Alex, Luke, and I just discussed this and decided we're going to scale back these changes given how big they turned out to be implementation-wise. I'm going to close this and open a new one next week.