Improve performance of split_scalar_matrix

[blegat]

@profview showed quite clearly that all the time was taken by allocating new empty vectors in these three lines:

MosekTools.jl/src/constraint.jl

Lines 54 to 56 in 650846d

	sd_row = Vector{Int32}[Int32[] for i in 1:length(m.sd_dim)]
	sd_col = Vector{Int32}[Int32[] for i in 1:length(m.sd_dim)]
	sd_coef = Vector{Float64}[Float64[] for i in 1:length(m.sd_dim)]

On @mipals benchmark ..._446.cbf:

function bench(n)
    file = "..._$n.cbf"
    model = @time JuMP.read_from_file(file)
    @time set_optimizer(model, Mosek.Optimizer)
    @time MOI.Utilities.attach_optimizer(backend(model))
end

bench(446)

Before the PR:

  1.203168 seconds (10.11 M allocations: 451.678 MiB, 4.00% gc time)
  0.000813 seconds (2.10 k allocations: 46.250 KiB)
  7.068828 seconds (282.64 M allocations: 18.978 GiB, 12.78% gc time)

After the PR:

  1.185881 seconds (10.11 M allocations: 451.678 MiB, 3.66% gc time)
  0.000877 seconds (2.10 k allocations: 46.250 KiB)
  0.074850 seconds (1.10 M allocations: 101.496 MiB, 7.87% gc time)

Closes #135