librustc: Make bare functions implement the FnMut trait.

This is done entirely in the libraries for functions up to 16 arguments.
A macro is used so that more arguments can be easily added if we need.
Note that I had to adjust the overloaded call algorithm to not try
calling the overloaded call operator if the callee is a built-in
function type, to prevent loops.

Closes #15448.

src/test/run-pass/bare-fn-implements-fn-mut.rs

@@ -0,0 +1,37 @@
+// Copyright 2014 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.
+
+#![feature(overloaded_calls)]
+
+use std::ops::FnMut;
+
+fn call_f<F:FnMut<(),()>>(mut f: F) {
+    f();
+}
+
+fn f() {
+    println!("hello");
+}
+
+fn call_g<G:FnMut<(String,String),String>>(mut g: G, x: String, y: String)
+          -> String {
+    g(x, y)
+}
+
+fn g(x: String, y: String) -> String {
+    x.append(y.as_slice())
+}
+
+fn main() {
+    call_f(f);
+    assert_eq!(call_g(g, "foo".to_string(), "bar".to_string()).as_slice(),
+               "foobar");
+}
+