Promotion rules

```julia
julia> one = FixedDecimal{Int,2}(1)
FixedDecimal{Int64,2}(1.00)

julia> one/3
FixedDecimal{Int64,2}(0.33)

julia> one/3.0
0.3333333333333333
```

Intuitively, I would expect `one/3 === one/3.0` since

```julia
julia> 1/3 === 1/3.0
true
```

Strictly speaking, `one/3` should probably also be a `Float64`, BUT...

I think users of this package might reasonably expect both

```julia
julia> one/3
FixedDecimal{Int64,2}(0.33)
```

and

```julia
julia> one/3.0
FixedDecimal{Int64,2}(0.33)
```

Otherwise, there would need to be a lot of conversion in the code that would kind of defeat its purpose. For example, if you wanted the result of `one/3.0` to remain a `FixedDecimal`, you'd need to do something like

```julia
> onethird = FixedDecimal{Int,2}(one/3.0)
```

I think we should either:

1. Make `one/3` also a `Float64` so `one/3 === one/3.0` or
1. Make `one/3.0` also a `FixedDecimal` so `one/3 === one/3.0`

I kind of prefer the latter, but in any case it currently seems inconsistent.

What do you think?