groupby function that returns a Dict of indices

groupby_dict.jl

# see https://discourse.julialang.org/t/allocation-groupby/7505
function groupby(v::AbstractVector{T}) where T
   d = Dict{T,Vector{Int}}()
   i = 1
   for x in v
       if !haskey(d, x)
           d[x] = [i]
       else
           push!(d[x], i)
       end
       i += 1
   end
   d
end