Error reporting

Fix-Its

rustc should output "fix-its" like clang does. Here are some ideas:

• Incorrect use of numeric literals.

    auto i = 0u;
    i += 3; // suggest "3u"
  

• Use of for where for each was meant.

    for (v in foo.iter()) // suggest "for each"
  

• Typos on record field names.

    fn f(rec(int foo, int bar) r) { ... }
    f(rec(foo=1, baz=2)); // suggest "bar"