Add support for numerator(::FD) and denominator(::FD)

[NHDaly]

We already support converting a FixedDecimal to a Rational, via:

FixedPointDecimals.jl/src/FixedPointDecimals.jl

Lines 324 to 326 in e8c9f3b

	function Base.convert(::Type{TR}, x::FD{T, f}) where {TR <: Rational, T, f}
	convert(TR, x.i // coefficient(FD{T, f}))::TR
	end

We should also provide numerator() and denominator(), as part of that interface. They might be implemented like this:

function Base.numerator(x::FixedDecimal{T,f})::T where {T, f}
    return value(x)
end
function Base.denominator(x::FixedDecimal{T,f})::T where {T, f}
    return coefficient(x)
end