unbroadcast for tuples

[mcabbott]

Fixes #975

[CarloLucibello]

I wonder if we can use dispatch on abstractarrays instead of this branching

[mcabbott]

What this should avoid is summing a 9x1 matrix over the 2nd dimension, and hence making a copy. whether this can plausibly matter for arrays short enough to become tuples, I don't know. The function does the same thing for arrays (just above) where it can matter:

julia> @btime sum($(rand(100,1,100)); dims=2);
  3.713 μs (8 allocations: 78.31 KiB)

julia> @btime sum($(rand(100,2,100)); dims=2);
  5.229 μs (8 allocations: 78.31 KiB)

julia> @btime reshape($(rand(100,1,100)), (100,100));
  17.117 ns (1 allocation: 64 bytes)