Report fields which are never read as dead.

Dead code analysis finds unused types and functions, but doesn't look at struct fields.

``` rust
struct Foo {
    bar: A,
    baz: B
}
fn foo() {
    let foo = Foo {bar: a, baz: b};
    foo.bar.do_something();
    // Never read foo.baz again. Compiler doesn't complain.
}
```

This could also be applied to tuple structs and enum variants.
