cc @stephentyrone

This is mostly expected. `456.789 as Double` has the exact value 456.788999999999987267074175179004669189453125, which is not exactly representable as `Decimal`, so it gets rounded to a representable `Decimal` value.

That second rounding appears to be not quite perfect (that's a Foundation bug), but even if Foundation were rounding correctly, the result would not be 456.789. It would be something like 456.78899999999998726707417517900466919 instead.

I wonder if a better solution would be to use the .description property of Double:

public init(_ value: Double) {
    self = Decimal(string: value.description) ?? Decimal.nan
}

and Decimal.description:

internal var doubleValue: Double {
    return Double(self.description) ?? Double.nan
} 

Whilst there will be a conversion cost going via a String, it may provide less surprising values overall.