Create a function to return the Nth number in the Fibonacci sequence as a string.
fibonacci(10) ➞ "55"
fibonacci(20) ➞ "6765"
fibonacci(30) ➞ "832040"
fibonacci(40) ➞ "102334155"
fibonacci(50) ➞ "12586269025"
fibonacci(60) ➞ "1548008755920"
Your function is expected to calculate numbers greater than the 64-bit unsigned integer limit where n can be as large as but not greater than 200.