Add heuristic choosing block or visual indent for unary op based on span

[topecongiro]

This PR closes #1327.
Input:

fn foo() {
    let hash: &Fn(&&Block) -> u64 = &|block| -> u64 {
                                         let mut h = SpanlessHash::new(cx);
                                         h.hash_block(block);
                                         h.finish()
                                     };
}

Output:

fn foo() {
    let hash: &Fn(&&Block) -> u64 = &|block| -> u64 {
        let mut h = SpanlessHash::new(cx);
        h.hash_block(block);
        h.finish()
    };
}

[nrc]

Looks good, thank you!