Write a function that returns the number of ways a person can climb n stairs, where the person may only climb 1 or 2 steps at a time.
To illustrate, if n = 4 there are 5 ways to climb:
[1, 1, 1, 1]
[2, 1, 1]
[1, 2, 1]
[1, 1, 2]
[2, 2]
ways_to_climb(1) ➞ 1
ways_to_climb(2) ➞ 2
ways_to_climb(5) ➞ 8
A staircase of height 0 should return 1.