Fillin missing functionality in ReservedVector and make it constexpr

Summary

Some typedefs and functions where missing in the ReservedVector implementation, like const_pointer and data(). Compared to std::array also some other functions could be added. Additionally, nearly all functions can be constexpr as in the std::array implementation.

Major Changes

• Mark almost all functions constexpr
• Mark almost all functions noexcept
• Add reverse iterators
• Change all iterators to std::array::[const_]iterator
• Add member functions push_back(T&&), emplace_back(Args&&...), cbegin(), rbegin(), crbegin(), cend(), rend(), crend(), at(), data(), fill(), swap()
• Use consistently size_type and value_type
• Use typedefs directly from std::array