Вильчинский - Лабораторная работа #1

You saw the interface Deque, as it exists in Java. It has the `getFirst` method, that throws an exception. `pool/peek` methods don't throw any exception and can be used in some cases w/o `try/catch/finally` blocks, so I suggest to add it into implementation. I'm going to merge this as is, and suggest you to add this methods in second lab (also persist throwing exception in `getFirst` method). Hope, you catch it now =).