Improve performance for recursion

[lucasccordeiro]

ESBMC-Python can verify this Python program in a few milliseconds, while ESBMC-Python-CPP takes many seconds:

def fib(n:int) -> int:
  if n <= 0:
    return 0
  elif n == 1:
    return 1
  else:
    return fib(n - 1) + fib(n - 2)

n:int = 10
result:int = fib(n)