Make accumulate and cumsum return Dictionary

[jariji]

I wish this kept the keys.

julia> let d = Dictionary([1,2,3], [10,20,30])
           cumsum(d)
       end
3-element Vector{Int64}:
 10
 30
 60

I can do it myself

julia> let d = Dictionary([1,2,3], [10,20,30])
           Dictionary(keys(d), cumsum(d))
       end
3-element Dictionary{Int64, Int64}
 1 │ 10
 2 │ 30
 3 │ 60

it would just be nicer if I didn't have to.

[andyferris]

That’s a good idea. Currently it must be falling back to the Base method.