Function clenshaw 

pub fn clenshaw(
    coeffs: &[ExactNum],
    x: &ExactNum,
    p: usize,
    rm: RoundingMode,
) -> ExactNum

Clenshaw recurrence for Σ_{k=0}^{n-1} c_k T_k(x) at precision p.

x is the Chebyshev variable on [-1, 1] (no interval map). Empty coeffs is zero. A non-finite coefficient or x yields NaN.