set NaN when constructing with non-decimal string
`Big<1, 2> value("a");`
will result in a valid object that evaluates to 0. Would you consider changing it so isNaN() evaluates to true? Because it literally is not a number.
`Big<1, 2> value("a");`
will result in a valid object that evaluates to 0. Would you consider changing it so isNaN() evaluates to true? Because it literally is not a number.