input: Write a function to calculate the nth Fibonacci number using tail recursion.
  assert Fibonacci(0) == 0
  assert Fibonacci(1) == 1
  assert Fibonacci(5) == 5
  assert Fibonacci(10) == 55
  assert Fibonacci(15) == 610
output: def Fibonacci(n, a=0, b=1):
      if n == 0:
          return a
      elif n == 1:
          return b
      else:
          return Fibonacci(n-1, b, a+b)