FD{BigInt} operations currently allocate more than necessary due to promotion.

Consider:
```julia
julia> @which FD{BigInt,2}(2) + 2
+(x::Number, y::Number)
@ Base promotion.jl:410

julia> @code_typed FD{BigInt,2}(2) + 2
CodeInfo(
1 ─ %1 = invoke Base.GMP.MPZ.set_si(10::Int64)::BigInt
│ %2 = invoke Base.GMP.bigint_pow(%1::BigInt, 2::Int64)::BigInt
│ %3 = invoke Base.GMP.MPZ.mul_si(%2::BigInt, y::Int64)::BigInt
│ %4 = Base.getfield(x, :i)::BigInt
│ %5 = invoke Base.GMP.MPZ.add(%4::BigInt, %3::BigInt)::BigInt
│ %6 = %new(FixedDecimal{BigInt, 2}, %5)::FixedDecimal{BigInt, 2}
└── return %6
) => FixedDecimal{BigInt, 2}

julia> @code_typed optimize=false FD{BigInt,2}(2) + 2
CodeInfo(
1 ─ %1 = Base.:+::Core.Const(+)
│ %2 = Base.promote(x, y)::Tuple{FixedDecimal{BigInt, 2}, FixedDecimal{BigInt, 2}}
│ %3 = Core._apply_iterate(Base.iterate, %1, %2)::FixedDecimal{BigInt, 2}
└── return %3
) => FixedDecimal{BigInt, 2}
```

If we instead had special-cased operators for `::FD{BigInt}, ::Integer` we could avoid the promotion and save an allocation.