[BUG] sort function doesn't return an array of correct type

**Describe the bug**
Newly added array `sort` and `sorted` return array of type `[Generic]`. This should be impossible. To fix this, we should add generics #978 .

**To Reproduce**
```
fun foo() {
return []
}

let a = foo() // `a` is of type `[Generic]`
```

**Expected behavior**
Clear error and a solution to use generics instead.

**Additional context**
#978 #966