wasmtime-py: Don't use "main.rs" for a wasm example so cargo doesn't test it (#464)

cargo test will test examples that have a "main.rs", but in this case,
the example requires manual compilation as a wasm module, and doesn't
build as a standalone program. Rename it to "demo.rs".

misc/wasmtime-py/examples/import/demo.rs

@@ -0,0 +1,11 @@
+extern "C" {
+    fn callback(s: *const u8, s_len: u32) -> u32;
+}
+
+#[no_mangle]
+pub extern "C" fn test() {
+    let msg = "Hello, world!";
+    unsafe {
+        callback(msg.as_ptr(), msg.len() as u32);
+    }
+}