Add support for asynchronous code

[CryZe]

This allows you to define an asynchronous main function instead of the poll based update function, which allows you to more easily keep state between individual ticks of the runtime. Unfortunately the most efficient implementation isn't possible yet on stable Rust, as we are blocked by the following two features:

• type_alias_impl_trait
• const_async_blocks

For now we have to use a workaround that is less efficient by calling the main function at runtime and allocating it onto a new WebAssembly page.

Here is a full example of how an auto splitter could look like using the async_main macro:

Usage on stable Rust:

async_main!(stable);

Usage on nightly Rust:

async_main!(nightly);

The asynchronous main function itself:

async fn main() {
    // TODO: Set up some general state and settings.
    loop {
        let process = Process::wait_attach("explorer.exe").await;
        process.until_closes(async {
            // TODO: Load some initial information from the process.
            loop {
                // TODO: Do something on every tick.
               next_tick().await;
            }
        }).await;
    }
}