If Expression

Like Rust, if statements can also be used as expressions, replacing the ? : conditional operators in other C-like languages.

[`if`] expressions can be disabled via [`Engine::set_allow_if_expression`][options].

// The following is equivalent to C: int x = 1 + (decision ? 42 : 123) / 2;
let x = 1 + if decision { 42 } else { 123 } / 2;
x == 22;

let x = if decision { 42 }; // no else branch defaults to '()'
x == ();