Add `.peek()`/`.top()` to sorters

[SSoelvsten]

By adding a .peek / .top() function to both types of sorters, we can simplify the levelized priority queue and also the reduce algorithm: currently they both store the peeked values in their own variables.

• [x] Add .top() to sorter<>.
• [ ] Simplify __reduce_level in src/adiar/internal/reduce.h by using .top().
• [ ] Simplify the bucketed levelized priority queue in src/adiar/internal/levelized_priority_queue.h by using .top().

Now that we are at it, we may as well include the following other functions of the priority queue.

• [ ] Add a .peek() alias for .top()
• [ ] Add a .pop() function to sorter<>