Initial reorg.

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

crates/wasi-common/src/macros.rs

@@ -0,0 +1,13 @@
+macro_rules! hostcalls {
+    ($(pub unsafe fn $name:ident($($arg:ident: $ty:ty,)*) -> $ret:ty;)*) => ($(
+            #[wasi_common_cbindgen::wasi_common_cbindgen]
+            pub unsafe fn $name($($arg: $ty,)*) -> $ret {
+                let ret = match crate::hostcalls_impl::$name($($arg,)*) {
+                    Ok(()) => crate::wasi::__WASI_ESUCCESS,
+                    Err(e) => e.as_wasi_errno(),
+                };
+
+                ret
+            }
+    )*)
+}