Define the **function**`is_permutation` that returns true if:
Define the **function**`is_permutation`, that returns `true` if the string `s1` is a permutation of `s2`.
- the string `s1` is a permutation of `s2`, otherwise it returns false.
`s1` is a permutation of `s2` if all the elements in `s1` appear the same number of times in `s2`, and all the characters in `s1` appear in `s2` even if they are in different order.
- `s1` is a permutation of `s2` if all the elements in `s1` appear the same number of times in `s2` and all the characters in `s1` appear in `s2` even if they are in different order.