Initial reorg.

This is largely the same as #305, but updated for the current tree.

crates/wasi-common/wasi-common-cbindgen/tests/mut_args.rs

@@ -0,0 +1,20 @@
+extern crate wasi_common_cbindgen;
+
+pub use wasi_common_cbindgen::wasi_common_cbindgen;
+
+#[wasi_common_cbindgen]
+fn mut_args(a: &mut usize) {
+    *a = *a + 1
+}
+
+fn main() {
+    let mut expected = Box::new(2);
+    mut_args(expected.as_mut());
+    let given = unsafe {
+        let given = Box::new(2);
+        let raw = Box::into_raw(given);
+        wasi_common_mut_args(raw);
+        Box::from_raw(raw)
+    };
+    assert_eq!(*given, *expected);
+}