Add a few more encoding tests

tests/ui/msg_send_not_encode.rs

@@ -0,0 +1,9 @@
+//! Test that return types that are not `Encode` are not accepted.
+use objc2::{class, msg_send};
+
+fn main() {
+    unsafe {
+        let cls = class!(NSData);
+        let _: Vec<u8> = msg_send![cls, new];
+    }
+}

tests/ui/msg_send_not_encode.stderr

@@ -0,0 +1,12 @@
+error[E0277]: the trait bound `Vec<u8>: Encode` is not satisfied
+   --> ui/msg_send_not_encode.rs:7:26
+    |
+7   |         let _: Vec<u8> = msg_send![cls, new];
+    |                          ^^^^^^^^^^^^^^^^^^^ the trait `Encode` is not implemented for `Vec<u8>`
+    |
+note: required by a bound in `send_message`
+   --> $WORKSPACE/objc2/src/message/mod.rs
+    |
+    |         R: Encode,
+    |            ^^^^^^ required by this bound in `send_message`
+    = note: this error originates in the macro `msg_send` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/ui/msg_send_only_message.rs

@@ -0,0 +1,6 @@
+//! Test that messages can only be sent to objects.
+use objc2::msg_send;
+
+fn main() {
+    unsafe { msg_send![1, new] };
+}

tests/ui/msg_send_only_message.stderr

@@ -0,0 +1,13 @@
+error[E0277]: the trait bound `{integer}: MessageReceiver` is not satisfied
+ --> ui/msg_send_only_message.rs:5:14
+  |
+5 |     unsafe { msg_send![1, new] };
+  |              ^^^^^^^^^^^^^^^^^ the trait `MessageReceiver` is not implemented for `{integer}`
+  |
+  = help: the following implementations were found:
+            <&'a T as MessageReceiver>
+            <&'a mut T as MessageReceiver>
+            <*const T as MessageReceiver>
+            <*mut T as MessageReceiver>
+          and 3 others
+  = note: this error originates in the macro `msg_send` (in Nightly builds, run with -Z macro-backtrace for more info)