Ban Stacked `if`'s

What it does

This lint should prohibit placing bool returning if statements as the expression inside another if statement. Currently, there appears to be no limit to the depth of stacked if's permitted by both rust fmt and clippy

Advantage

• Stacked if's are hard to read

Drawbacks

• Ternary-style expressions couldn't be used inside an if statement, but I personally think that's poor form

Example

if if if a == b {
    b == c
} else {
    a == c
} {
    a == d
} else {
    c == d
} {
    println!("True!");
} else {
    println!("False!");
}

Could be written as:

let expression_1 = if a == b {
    b == c
} else {
    a == c
};

let expression_2 = if expression_1 {
    a == d
} else {
    c == d
};

if expression_2  {
    println!("True!");
} else {
    println!("False!");
}