A slightly more specific conversion trait for classes which extend ScalaNumber (which excludes value classes.)
Should only be called after all known non-primitive types have been excluded. This method won't dispatch anywhere else after checking against the primitives to avoid infinite recursion between equals and this on unknown "Number" variants.
Additionally, this should only be called if the numeric type is happy to be converted to Long, Float, and Double. If for instance a BigInt much larger than the Long range is sent here, it will claim equality with whatever Long is left in its lower 64 bits. Or a BigDecimal with more precision than Double can hold: same thing. There's no way given the interface available here to prevent this error.