Fix Cargo warnings using latest Rust

This fixes a few warnings that will be produced when using the latest
versions of Rust and Cargo.

vm/src/mailbox.rs

@@ -72,7 +72,7 @@ impl Mailbox {
         if let Some(pointer) = self.internal.pop_front() {
             self.amount.fetch_sub(1, Ordering::AcqRel);
-            return (pointer.is_mailbox(), Some(pointer));
+            (pointer.is_mailbox(), Some(pointer))
         } else {
             (false, None)
         }

vm/src/prefetch.rs

@@ -35,8 +35,6 @@ pub fn prefetch_read<T>(pointer: *const T) {
                 arch_impl::_MM_HINT_NTA,
             );
         }
-
-        return;
     }
 }

vm/src/vm/io.rs

@@ -35,7 +35,7 @@ macro_rules! file_mode_error {
 pub fn io_read(
     state: &RcState,
     process: &RcProcess,
-    stream: &mut Read,
+    stream: &mut dyn Read,
     buffer: &mut Vec<u8>,
     amount: ObjectPointer,
 ) -> Result<ObjectPointer, RuntimeError> {