unstable.Queue: new class (not implemented)

FIFO: Interface for First In First Out data structures
Queue: Simple implementation FIFO

TODO: implement Queue

unstable.Stack: new unstable class

Last In First Out (LIFO) implementation as simple as possible.

TODO: add tests

NumberUtilTest: remove unnecessary operations

NumberUtilTest: add unordered property to stream

NumberUtilTest: fix indentation

NumberUtilTest: use Stream API for tests

NumberUtil.isPrime(): use IntStream.iterate()

NumberUtil.isPrime(): simplify lambda expression

NumberUtil.isPrime(): implement functionally

NumberUtil.isPrime(): implement completely

NumberUtil: new class

Initialize maven

app: org.example.app.MessageUtils: change greeting

remove gradle.properties

org.gradle.console is user related setting. Not necessary for project.

README.md: mention gradle build

use gradle

kthr.SList: rename from kthr.SignlyLinkedList

kthr.SinglyLinkedList: make final class

kthr.SinglyLinkedList: add deleteHead

kthr.SinglyLinkedList: add toString()