fix deprecation of std::iterator (#7332)

fix deprecation of std::iterator (#7332)

When compiling cvc5 with clang-13, it will emit lots of warnings of usages of `std::iterator` as it's deprecated since C++17.
The recommended implementation of iterators is to manually define the following five types:
```
template< class Iter >
struct iterator_traits;

difference_type	        Iter::difference_type
value_type	        Iter::value_type
pointer	                Iter::pointer
reference	        Iter::reference
iterator_category	Iter::iterator_category

```
And the iterator-related types could be accessed by for example `typename std::iterator_traits<Iter>::value_type value`.

This pull request performs the fix, and the program is semantically equivalent before and after the fix.

References:
https://en.cppreference.com/w/cpp/iterator/iterator_traits
https://en.cppreference.com/w/cpp/iterator/iterator