StreamEx 0.8.0

• [#244] Added: StreamEx.toMutableList and StreamEx.toMutableSet.
  Removed mutability guarantee for toList and toSet.
  Added temporary system property streamex.default.immutable (set it to 'true' to test with immutable toList and toSet)
• [#238] Added: valuesToMap, valuesToSortedMap, valuesToNavigableMap methods to StreamEx
• [#250] Fixed: EntryStream.withoutKeys and EntryStream.withoutValues declared as @SafeVarargs
• [#251] Changed: public classes like StreamEx and EntryStream declared as final now.
• [#246] Added: Proper module-info.class, now it's full-fledged JPMS module
• [#247] Fixed: now it's possible to build using Java 16 and Java 17 (though reference builds are created via Java 11)
• [#91] Changed: API tests moved to the separate package.
• [#185] Added: EntryStream.withoutKeys and EntryStream.withoutValues.
• Minor documentation fixes