v0.2.1 Rename to rand-unique and exact size iterating

• Implement RandomSequence<usize>.
• Implement Iterator::size_hint, with a fix in v0.2.1.
• Implement ExactSizeIterator on u8, u16, and u32. Cannot be implemented on usize and u64 as the length of the iterator is usize::MAX + 1.
• RandomSequence::next() and RandomSequence::prev() now return Option<T> and terminate at the end of the sequence instead of wrapping.
• Add RandomSequence::wrapping_next() and RandomSequence::wrapping_prev().
• RandomSequence::index() returns Option<T>
• Add RandomSequence::exhausted() -> bool and RandomSequence::set_index() methods.