Restore PrependSpliterator

[amaembo]

While TailConcatSpliterator is implemented (see #55) and useful, the PrependSpliterator which allows adding exactly one element at the beginning of the stream is very important use case for headTail scenarios (see #54). As recursively defined operations may use prepend for every single element, it's significant allocation pressure to create TailConcatSpliterator and ArraySpliterator every time. Replacing them both with simple PrependSpliterator reduces allocation and improves overall performance of recursive headTail operations.

• Implement StreamEx.prepend(T) and StreamEx.append(T) (for symmetry)
• Implement PrependSpliterator
• JavaDoc for new methods
• Tests
• Changes