Add powf

[asceenl]

High Koute,

First of all, I would like to express my thanks for publishing this lib. It is a nice piece of work. I missed one part, that is a powf function, for which I implemented it as follows. It is quite simple in implementation:

/// Raise `a` to the power `b`
const fn powf(a: F64, b: F64) -> F64 {
 softfloat::F64::exp(b.mul(a.ln()))
}

Maybe worth to add it?

Best regards,

Anne de Jong