Immutable EnumMap

[marc-mabe]

• added mutable methods
  • add($enumerator, $value)
  • addIterable(iterable $map)
  • remove($enumerator)
  • removeIterable(iterable $enumerators)
  • (These methods are added for consistent naming - see #120)
• added immutable methods
  • with($enumerator, $value)
  • withIterable(iterable $map)
  • without($enumerator)
  • withoutIterable(iterable $enumerators)
• other changes
  • added get($enumerator) : returns mapped data value or exception if missing
  • offsetGet($enumerator) now returns null if missing - this change is done to be consistent with offsetExists($enumerator) which gets called by isset and should follow this behavior