Function chebyshev_eval 

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

Evaluate the Chebyshev expansion of coeffs at x ∈ [a, b].

Maps x to [-1, 1] and calls clenshaw. A bad interval or non-finite input is NaN.