Fix rrule_via_ad return type

[mcabbott]

This ought to return a tuple; what it returns now works in some contexts but not JuliaDiff/ChainRules.jl#644 .

julia> using Diffractor, ChainRulesCore

julia> ret = rrule_via_ad(Diffractor.DiffractorRuleConfig(), x -> log(exp(x)), 2)
Diffractor.OpticBundle{Float64}(2.0, (::Any)::Tuple{ZeroTangent, Any}->◌)

julia> y, bk = ret;  # ok

julia> Tuple(ret)
ERROR: MethodError: no method matching length(::Diffractor.OpticBundle{Float64})

Originally added in #45, thus my fault...

[JeffBezanson]

That length method for OpticBundle should be added.

[mcabbott]

Is it ok to merge this?