Add fun text-to-speech example

Here's to hoping it actually works, I don't have a new enough system to actually test it

objc2/examples/talk_to_me.rs

@@ -0,0 +1,38 @@
+use objc2::ffi::NSUInteger;
+use objc2::rc::{Id, Owned, Shared};
+use objc2::runtime::Object;
+use objc2::{class, msg_send};
+use std::ffi::c_void;
+use std::ptr::NonNull;
+
+#[link(name = "AVFoundation", kind = "framework")]
+extern "C" {}
+
+// Only works on macOS >= 10.15 or iOS > 7.0
+fn main() {
+    let text = "Hello from Rust!";
+
+    let string: *const Object = unsafe { msg_send![class!(NSString), alloc] };
+    let string = unsafe {
+        msg_send![
+            string,
+            initWithBytes: text.as_ptr() as *const c_void,
+            length: text.len(),
+            encoding: 4 as NSUInteger, // UTF8_ENCODING on macOS / iOS
+        ]
+    };
+    let string: Id<Object, Shared> = unsafe { Id::new(NonNull::new(string).unwrap()) };
+
+    let synthesizer: *mut Object = unsafe { msg_send![class!(AVSpeechSynthesizer), new] };
+    let synthesizer: Id<Object, Owned> = unsafe { Id::new(NonNull::new(synthesizer).unwrap()) };
+
+    let utterance: *mut Object = unsafe { msg_send![class!(AVSpeechUtterance), alloc] };
+    let utterance: *mut Object = unsafe { msg_send![utterance, initWithString: &*string] };
+    let utterance: Id<Object, Owned> = unsafe { Id::new(NonNull::new(utterance).unwrap()) };
+
+    // let _: () = unsafe { msg_send![utterance, setVolume: 90.0f32 };
+    // let _: () = unsafe { msg_send![utterance, setRate: 0.50f32 };
+    // let _: () = unsafe { msg_send![utterance, setPitchMultiplier: 0.80f32 };
+
+    let _: () = unsafe { msg_send![synthesizer, speakUtterance: &*utterance] };
+}