Add support for asynchronous code

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](rust-lang/rust#63063)
- [const_async_blocks](rust-lang/rust#85368)

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:
```rust
async_main!(stable);
```

Usage on nightly Rust:
```rust

async_main!(nightly);
```

The asynchronous main function itself:
```rust
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;
    }
}
```

README.md

@@ -1,15 +1,34 @@
 # <img src="https://raw.githubusercontent.com/LiveSplit/LiveSplit/master/LiveSplit/Resources/Icon.png" alt="LiveSplit" height="42" width="45" align="top"/> asr
 
-Helper crate to write auto splitters for LiveSplit One's auto splitting runtime.
 
-## Example
+Helper crate to write auto splitters for LiveSplit One's auto splitting
+runtime.
+
+There are two ways of defining an auto splitter.
+
+## Defining an `update` function
+
+You can define an `update` function that will be called every frame. This is
+the simplest way to define an auto splitter. The function must have the
+following signature:
+```rust
+#[no_mangle]
+pub extern "C" fn update() {}
+```
+
+The advantage of this approach is that you have full control over what
+happens on every tick of the runtime. However, it's much harder to keep
+state around as you need to store all state in global variables as you need
+to return out of the function on every tick.
+
+### Example
 
 ```rust
 #[no_mangle]
 pub extern "C" fn update() {
-    if let Some(process) = Process::attach("Notepad.exe") {
+    if let Some(process) = Process::attach("explorer.exe") {
         asr::print_message("Hello World!");
-        if let Ok(address) = process.get_module_address("Notepad.exe") {
+        if let Ok(address) = process.get_module_address("explorer.exe") {
             if let Ok(value) = process.read::<u32>(address) {
                 if value > 0 {
                     asr::timer::start();
@@ -20,6 +39,74 @@ pub extern "C" fn update() {
 }
 ```
 
+## Defining an asynchronous `main` function
+
+You can use the `async_main` macro to define an asynchronous `main`
+function.
+
+Similar to using an `update` function, it is important to constantly yield
+back to the runtime to communicate that the auto splitter is still alive.
+All asynchronous code that you await automatically yields back to the
+runtime. However, if you want to write synchronous code, such as the main
+loop handling of a process on every tick, you can use the
+`next_tick` function to yield back to the runtime and
+continue on the next tick.
+
+The main low level abstraction is the `retry` function, which wraps any code
+that you want to retry until it succeeds, yielding back to the runtime between
+each try.
+
+So if you wanted to attach to a Process you could for example write:
+
+```rust
+let process = retry(|| Process::attach("MyGame.exe")).await;
+```
+
+This will try to attach to the process every tick until it succeeds. This
+specific example is exactly how the `Process::wait_attach` method is
+implemented. So if you wanted to attach to any of multiple processes, you could
+for example write:
+
+```rust
+let process = retry(|| {
+   ["a.exe", "b.exe"].into_iter().find_map(Process::attach)
+}).await;
+```
+
+### Example
+
+Here is a full example of how an auto splitter could look like using the
+`async_main` macro:
+
+Usage on stable Rust:
+```rust
+async_main!(stable);
+```
+
+Usage on nightly Rust:
+```rust
+#![feature(type_alias_impl_trait, const_async_blocks)]
+
+async_main!(nightly);
+```
+
+The asynchronous main function itself:
+```rust
+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;
+    }
+}
+```
+
 ## License
 
 Licensed under either of