Move callback guest out of direct examples dir

examples/callback-guest/README.md

@@ -0,0 +1,5 @@
+# Call back guest
+
+This is part of the `callback` example.  This Wasm module passes host imports and its own functions to the Wasm host to execute.
+
+See `examples/callback.rs` for the host

examples/callback-guest/callback-guest.rs

@@ -0,0 +1,24 @@
+extern "C" {
+    fn call_guest_fn(f: u32) -> u32;
+    fn call_guest_fn2(f: u32) -> u32;
+    fn host_callback() -> u32;
+}
+
+#[no_mangle]
+fn test_callback() -> u32 {
+    42
+}
+
+#[no_mangle]
+fn test_callback2() -> u32 {
+    45
+}
+
+fn main() {
+    unsafe { call_guest_fn(test_callback as usize as u32) };
+    unsafe { call_guest_fn(host_callback as usize as u32) };
+    unsafe { call_guest_fn(test_callback2 as usize as u32) };
+    unsafe { call_guest_fn2(test_callback2 as usize as u32) };
+    unsafe { call_guest_fn2(test_callback as usize as u32) };
+    unsafe { call_guest_fn2(host_callback as usize as u32) };
+}