ported/tanh

pub fn tanh_kernel(n: f64) -> f64 {
    // W111: small arguments reuse the cancellation-safe expm1 pair for the
    // denominator too. The public COSH result is observably different there.
    let denominator = if n.abs() < 1.0 {
        let sum = crate::ported::excel_numeric::excel_x87_add(
            crate::ported::excel_numeric::excel_expm1_internal(n),
            crate::ported::excel_numeric::excel_expm1_internal(-n),
        );
        crate::ported::excel_numeric::excel_x87_add(sum, 2.0) / 2.0
    } else {
        crate::ported::cosh::cosh_kernel(n)
    };
    crate::ported::excel_numeric::excel_x87_div(
        crate::ported::sinh::sinh_kernel(n),
        denominator,
    )
}