Initial reorg.

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

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

@@ -0,0 +1,12 @@
+extern crate wasi_common_cbindgen;
+
+pub use wasi_common_cbindgen::wasi_common_cbindgen;
+
+#[wasi_common_cbindgen]
+fn val_args(a: usize, b: usize) -> usize {
+    a + b
+}
+
+fn main() {
+    assert_eq!(unsafe { wasi_common_val_args(1, 2) }, val_args(1, 2));
+}