@let and @for should support type annotations

[lambda-fairy]

The following syntax should work, but doesn't:

html! {
    @let foo: Bar = baz() {
        // ...
    }
}

See #72.

[trombonehero]

The same is probably true of @for as well, e.g.:

@for u: User in self.0.as_ref() {
    tr {
        td (u.name)
        td (u.email)
        /* ... */
    }
}

[lambda-fairy]

The following code works on master:

#[test]
fn let_type_ascription() {
    let s = html! {
        @let mut x: Box<Iterator<Item=u32>> = Box::new(vec![42].into_iter()) {
            "I have " (x) " cupcakes!"
        }
    }.into_string();
    assert_eq!(s, "I have 42 cupcakes!");
}

I have added this as a test in 4a73399.

Type ascription in for patterns isn't supported in Rust proper, so I'll consider that a wontfix. See rust-lang/rfcs#354.