pow: Round negative to zero.

[github-actions]

On 2022-05-31 @recmo wrote in c1a0c34 “Merge pull request #97 from recmo/isqrt”:

Round negative to zero.
Self::BITS ~< 2^52 and so fits f64.

    #[must_use]
    pub fn approx_pow2(exp: f64) -> Option<Self> {
        const LN2_1P5: f64 = 0.584_962_500_721_156_2_f64;
        const EXP2_63: f64 = 9_223_372_036_854_775_808_f64;

        // FEATURE: Round negative to zero.
        #[allow(clippy::cast_precision_loss)] // Self::BITS ~< 2^52 and so fits f64.
        if exp < LN2_1P5 {
            if exp < -1.0 {
                return Some(Self::ZERO);
            }
            return Self::try_from(1).ok();

From src/pow.rs:101