add wasi

wasi/src/main.rs

@@ -0,0 +1,10 @@
+use std::fs::File;
+use std::io::prelude::*;
+
+fn main() -> std::io::Result<()> {
+    let mut file = File::create("foo.txt")?;
+    file.write_all(b"Hello, world!")?;
+    println!("Hello, world!");
+    Ok(())
+}
+