Improve readability

[bokkypoobah]

As suggested in #1

[bokkypoobah]

next(...)
  x -> target
  y -> cursor

prev(...)
  x -> target
  y -> cursor

insert(...)
  x -> probe
  y -> cursor
  z -> key

remove(...)
  x -> probe
  y -> cursor
  z -> key
  
rotateLeft(...)
  x -> key
  y -> cursor
  _parent -> keyParent
  yLeft -> cursorLeft

rotateRight(...)
  x -> key
  y -> cursor
  _parent -> keyParent
  yRight -> cursorRight

insertFixup(...)
  y -> cursor
  z -> key
  zParent -> keyParent

removeFixup(...)
  x -> key
  xParent -> keyParent
  w -> cursor

SENTINEL -> EMPTY

Add isEmpty()
Add getEmpty()

[bokkypoobah]

Gas increased slightly for insert after part 2 remove(...), most likely due to part 1 changes in insert(...) with the exist(...) replacing the duplicate check

Gas Before:

minGasUsedInsert=68318
averageGasUsedInsert=112496.21875
maxGasUsedInsert=167842
minGasUsedRemove=30542
averageGasUsedRemove=80300.5
maxGasUsedRemove=190402

Gas After:

minGasUsedInsert=68891
averageGasUsedInsert=113027.78125
maxGasUsedInsert=168337
minGasUsedRemove=27458
averageGasUsedRemove=66072.125
maxGasUsedRemove=169772

[bokkypoobah]

Gas Before:

minGasUsedInsert=68891
averageGasUsedInsert=113027.78125
maxGasUsedInsert=168337
minGasUsedRemove=27458
averageGasUsedRemove=66072.125
maxGasUsedRemove=169772

Gas After:

minGasUsedInsert=68891
averageGasUsedInsert=113027.78125
maxGasUsedInsert=168337
minGasUsedRemove=27375
averageGasUsedRemove=65965.78125
maxGasUsedRemove=169605