Issues with nested trace about cassette.jl

[jrevels]

Looks like this was fixed by #28 (if nothing is the expected value, at least, reopen if not) :

julia> versioninfo()
Julia Version 0.7.0-DEV.3436
Commit 26a2ea2 (2018-01-17 15:46 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin16.7.0)
  CPU: Intel(R) Core(TM) i5-4288U CPU @ 2.60GHz
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Haswell)
  LAPACK: libopenblas64_
  LIBM: libopenlibm
  LLVM: libLLVM-3.9.1 (ORCJIT, haswell)
Environment:

julia> using Cassette

julia> using Cassette: @context, @primitive, overdub

julia> @context Trace

julia> @context BTrace

julia> @primitive Trace (::typeof(broadcast))(f, args...) = overdub(BTrace, f)(args...)

julia> @primitive BTrace (::typeof(+))(args...) = nothing

julia> # Works
       overdub(Trace, (a,b) -> a .+ b)([1,2,3],[4,5,6])

julia> # Segfaults
       overdub(Trace, x -> x .+ x)([1,2,3])

julia> overdub(Trace, (a,b) -> a .+ b .+ b)([1,2,3],[4,5,6])