Fix for MutableLinkedList append!

[synthfi]

See #739 for example of the segfault that I'm seeing

• Updates Base.append!(l1::MutableLinkedList{T}, l2::MutableLinkedList{T}) where T so that the resulting object doesn't print endlessly to the screen, and that collect doesn't segfault the julia session
  • adds more lines to update node next and prev pointers
• Adds a test in the append section so that the second argument is checked for being empty

with these changes, I'm seeing this work without segfaulting

(@v1.9) pkg> activate .
  Activating project at `~/repos/DataStructures.jl`

julia> using DataStructures

julia> l1, l2 = MutableLinkedList{Int}(1,2), MutableLinkedList{Int}(3,4)
(MutableLinkedList{Int64}(1, 2), MutableLinkedList{Int64}(3, 4))

julia> append!(l1, l2)
MutableLinkedList{Int64}(1, 2, 3, 4)

julia> collect(l1)
4-element Vector{Int64}:
 1
 2
 3
 4

julia> l2
MutableLinkedList{Int64}()

[laborg]

Fixes the problematic behaviour. For 1.0 it should be decided if append! should mutate the second list...

[oxinabox]

It should not, since we want to match the behavour of append!ing two Vectors

[synthfi]

I'll change it to do a copy of the second list

[oxinabox]

It's unfortunate since that's very expensive with a linked list.
Its too bad these are double-y linked lists...

Linked lists are such a can of worms