Suggestion: add atan2 function

[alteous]

I noticed it was missing from d4647ad in the UnaryOperation enum:

/// Operations that can be applied to a value in memory, requiring one operand.
#[derive(Debug, Deserialize, Serialize, Clone, Copy, PartialEq, Eq)]
pub enum UnaryOperation {
    /// Logical negation
    Not,
    /// Flip the sign of a signed number
    Neg,
    /// Get the absolute value of a number
    Abs,
    /// Arc cosine
    Acos,
    /// Arc sine
    Asin,
    /// Arc tangent
    Atan,
    /// Ceiling
    Ceil,
    /// Cosine
    Cos,
    /// Floor,
    Floor,
    /// Natural logarithm
    Ln,
    /// Logarithm base 10
    Log10,
    /// Logarithm base 2
    Log2,
    /// Sine
    Sin,
    /// Square root
    Sqrt,
    /// Tangent
    Tan,
    /// Convert radians to degrees
    ToDegrees,
    /// Convert degrees to radians
    ToRadians,
}