Add box mut dyn trait regression test (#108)

This resolves issue #34

Co-authored-by: Mark R. Tuttle <mrtuttle@amazon.com>

rust-tests/cbmc-reg/FatPointers/boxmuttrait.rs

@@ -0,0 +1,14 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+use std::io::{sink, Write};
+
+fn main() {
+    let mut log: Box<dyn Write + Send> = Box::new(sink());
+    let dest: Box<dyn Write + Send> = Box::new(log.as_mut());
+
+    let mut log2: Box<dyn Write + Send> = Box::new(sink());
+    let buffer = vec![1, 2, 3, 5, 8];
+    let num_bytes = log2.write(&buffer).unwrap();
+    assert!(num_bytes == 5);
+}