Implement `Tree(...).concat(t :: Tree)`

[OxSon]

The main structure of the code is there, now, but it is not yet working as intended in some ways. The result of some_tree.concat(other_tree).to_list() will always be as expected, but there are performance issues lurking and a lack of thorough testing involving trees of varied sizes as well as trees created through complex-ish series of operations including concats, appends, sets, etc.

• [x] fix #5 may not need fixing, see comments on #5
• [x] needs more thorough testing
  • [x] #7
  • [x] randomized tests
  • [x] test very large trees

[OxSon]

done