…avr open for extensibility.

Improves javadoc.

Merge latest changes from vavr project.

# Conflicts:
#	README.md

Improve javadoc of JMH benchmarks.
Update to 'org.scala-lang:scala-library:2.13.11-M1'.

Lump all utility classes into a single class, because Java 8 does not support the notion of non-exported packages.

…apTest.java.

Changes:

build.gradle
- Temporarily raises Java version from 8 to 17, because we are porting code from JHotDraw 8 which is on 17. We intend to change this back to Java 8.

ChampAbstractChampSpliterator.java,
ChampAbstractTransientChampSet.java,
ChampBitmapIndexedNode.java,
ChampChangeEvent.java,
ChampHashCollisionNode.java,
ChampIdentityObject.java,
ChampIteratorAdapter.java,
ChampListHelper.java,
ChampMutableBitmapIndexedNode.java,
ChampMutableHashCollisionNode.java,
ChampNode.java,
ChampNodeFactory.java,
ChampSequencedData.java,
ChampSequencedElement.java,
ChampSequencedEntry.java,
ChampSequencedVectorSpliterator.java,
ChampSpliterator.java,
ChampTombstone.java
- Classes with prefix "Champ" provide the CHAMP trie data structure and operations that act on it.
- Maybe we lump them all together into a ChampTrie class (similar to BitMappedTrie). However, for now, it is much easier to have separate source files for each class.

HashSet.java
- Replaces its internal representation from HashArrayMappedTrie to a CHAMP trie.
- This class extends from ChampBitmapIndexedNode rather than using composition. This helps to reduce memory usage for small collections (we save 16 bytes), and this improves the performance (we save one level of indirection).

Changes:

HashSet.java
- Override spliterator().

Changes:

HashMap.java
- Replaces its internal representation from HashArrayMappedTrie to a CHAMP trie.

ChampChangeEvent.java
- Adds method isAdded().

HashSet.java
- Adds javadoc.
- Renames method updateFunction to updateElement.

HashMapTest.java
- Changes the expected result of 2 unit tests. I believe, that the test expectations were wrong.

WIP: Improve performance with transience.

Improve performance of Collections.retainAll.

There appears to be a bug in transient 'update' methods in CHAMP trie nodes, because it is not faster than non-transient 'update'.
Transient 'remove' in CHAMP trie nodes is faster than the non-transient 'remove' - so that seems to be correct.

…k results.

# Conflicts:
#	build.gradle
#	src/main/java/io/vavr/collection/HashMap.java
#	src/main/java/io/vavr/collection/HashSet.java
#	src/main/java/io/vavr/collection/LinkedHashMap.java
#	src/main/java/io/vavr/collection/LinkedHashSet.java

… therefore has to be upper case.