Ensure `FutureWriter` always writes

[alexcrichton]

Discussed some time ago, forgotten, and then reminded today, the expectation is that at the component model level dropping a writable-end future will be a trap if nothing was actually written. Modeling this in Rust without actually surfacing traps will be done with a few changes:

• When a FutureWriter<T> is constructed an additional parameter default: fn() -> T will be required. This will be stored within the FutureWriter<T> return value.
• When a FutureWriter<T> is dropped without recording a successful write then a task will be spawned which actually performs the write (e.g. calls the default and then performs the write for you).

This needs to be implemented in the Rust bindings.