Function verify_triangle_inequality 

pub fn verify_triangle_inequality<F>(
    metric: F,
    x: &[u8],
    y: &[u8],
    z: &[u8],
    tolerance: f64,
) -> bool
where
    F: Fn(&[u8], &[u8]) -> f64,

Verify triangle inequality: d(x,z) ≤ d(x,y) + d(y,z).