Broadcasting and constructors

There seems to be something weird going on when broadcasting over `Real`:

```julia
julia> using Tracker, Distributions

julia> m = first(param(zeros(1)))
0.0 (tracked)

julia> s = first(param(ones(1)))
1.0 (tracked)

julia> typeof(Normal.(m, s))
Tracker.Tracked{Normal{Float64}}
```

But the strange thing is that the following seems to work just fine:

```julia
julia> struct TwoFields{T1,T2}
x::T1
y::T2
end

julia> TwoFields.(first(param(zeros(1))), first(param(zeros(1))))
TwoFields{Float64, Float64}(0.0, 0.0)
```

Possibly related to: #65

Start by reproducing the Julia examples for Normal.(m, s) and TwoFields.(...). Read the broadcasting and constructor behavior implicated by the examples, then compare with issue #65. Done means the Normal broadcast no longer produces an unexpected Tracker.Tracked result while the demonstrated TwoFields behavior remains correct.

Written by the indexing model from the issue text.