Diff algorithm preforms poorly on prepend

[OlivierBlanvillain]

The diff algorithm works fine when appending:

val a = 'div (
  'h2 /= "", 'h1 /= "", 'h2 /= "", 'h1 /= "", 'h2 /= "", 'h1 /= "", 'h2 /= "")

val b = 'div (
  'h2 /= "", 'h1 /= "", 'h2 /= "", 'h1 /= "", 'h2 /= "", 'h1 /= "", 'h2 /= "",
  'h1 /= "new")
  
VDom.changes(a, b) == List(SetAttr(0,"h1","new",false))

But seams to preform poorly when prepending:

val a = 'div (
  'h2 /= "", 'h1 /= "", 'h2 /= "", 'h1 /= "", 'h2 /= "", 'h1 /= "", 'h2 /= "")

val b = 'div (
  'h1 /= "new",
  'h2 /= "", 'h1 /= "", 'h2 /= "", 'h1 /= "", 'h2 /= "", 'h1 /= "", 'h2 /= "")

VDom.changes(a, b) == List(
  SetAttr(0,"h2","",false),
  RemoveAttr(0,"h2",""false),
  SetAttr(0,"h1","",false),
  RemoveAttr(0,"h1",""false),
  SetAttr(0,"h2","",false),
  RemoveAttr(0,"h2",""false),
  SetAttr(0,"h1","",false),
  RemoveAttr(0,"h1",""false),
  SetAttr(0,"h2","",false),
  RemoveAttr(0,"h2",""false),
  SetAttr(0,"h1","",false),
  RemoveAttr(0,"h1",""false),
  SetAttr(0,"h2","",false),
  RemoveAttr(0,"h2",""false),
  SetAttr(0,"h1","new",false)
)

FWIW the kivi implementation seams to hande these cases while remaining reasonably simple

[fomkin]

Solved #120