Fix type instability due to mapslices in colorvec partial generation

[ChrisRackauckas]

using BenchmarkTools, SparseArrays, SparseDiffTools, ForwardDiff
function f!(H, x)
    for n in 1:length(H)
        if isodd(n)
            H[n] = 2*x[1]
        else
            H[n] = 3*x[2]
        end
    end
end
H = zeros(10000)
x = 5.0 * ones(200)
f!(H, x)

pattern = sparse(vcat(collect(1:2:10000), collect(2:2:10000)), vcat(repeat([1], 5000), repeat([2], 5000)), 1, length(H), length(x))
jac = Float64.(sparse(pattern))
colors = matrix_colors(jac)
@btime forwarddiff_color_jacobian!(jac, f!, x, colorvec = colors);

goes from 9.993 ms (136398 allocations: 4.46 MiB) to 605.200 μs (696 allocations: 284.89 KiB)

[ChrisRackauckas]

Tested locally due to SparsityDetection bug and passes, so merging.