Forbid modifications of strings in the compiler

This disallows `str[0] = foo` along with `foo = &mut str[i]` to prevent strings
from being modified at runtime (except possibly through the `str` module)

Closes #8891

src/test/compile-fail/unsafe-modifying-str.rs

@@ -0,0 +1,18 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    let mut s = ~"test";
+    s[0] = 3; //~ ERROR: not allowed
+    s[0] += 3; //~ ERROR: not allowed
+    {
+        let _a = &mut s[0]; //~ ERROR: not allowed
+    }
+}