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"