Release v5.6.0

• Add byNameOrNull extension on enum.
• Add toFixedString extension on int.
• [BREAKING] Fix flatMap extension to behaving correctly.
• Iterable extensions:
  • Add flatMapNotNull extension.
  • Add mapNotNull extension.
  • Add mapNotNullIndexed extension.
  • Add 'flattenedNotNull' extension.
• Add << operator extension for List.
• randomInt function improvements:
  • add seed option.
  • Change default max value to max int value allowed by Random.
• Add reversed, findByValue and findByValueOrNull map extensions.
• Add plus, minus, multiply, divide and mod extensions for num.