* Add `const` keyword to the `operator-(const uoffset_t &)` function in
`VectorIterator`

* Support reverse iterator in Vector

Introduced a new VectorReverseIterator type. We cannot directly use
`std::reverse_iterator<VectorIterator>` because the signature of
`operator*` and `operator->` in the VectorIterator class are not
standard signatures.

Also added `rbegin()`, `rend()`, `cbegin()`, `cend()`, `crbegin()`
and `crend()` in the Vector class.