Function secant 

pub fn secant<F: Fn(f64) -> f64>(
    f: F,
    x0: f64,
    x1: f64,
    tol: f64,
    max_iter: usize,
) -> RootResult

Secant method root finding.

Uses two initial guesses x0 and x1 without requiring the derivative. Converges super-linearly (order ≈ 1.618) for smooth functions.