Given the size, return a square matrix of numbers in spiral order.
The matrix should be filled with natural numbers, starting from 1 in the top-left corner, increasing in an inward, clockwise spiral order, like these examples:
1 2 3
8 9 4
7 6 5
1 2 3 4
12 13 14 5
11 16 15 6
10 9 8 7