add more support for Base.setindex

[dlfivefifty]

At the moment we only support setting a single index, but I think eg the following should be supported:

julia> Base.setindex(Zeros(5,5), randn(2,2), 1:2, 1:2)
ERROR: MethodError: no method matching setindex(::Zeros{Float64, 2, Tuple{…}}, ::Matrix{Float64}, ::UnitRange{Int64}, ::UnitRange{Int64})

[dlfivefifty]

In particular, it will be useful for making structured matrices. Eg. if we setindex a banded matrix into a Zeros it will make a banded matrix.