UncheckedArray and closure issue

[Araq]

Start by not capturing `elements` in:
    
    
    (index.with_length length).sort do (a, b: int) -> int:
        elements[a].cmp elements[b]
    
    
    
    Run

And in general it helps to know the language. For example `cast[ptr 
UncheckedArray[T]](cast[pointer](alloc(n * sizeof(T))))` should be `cast[ptr 
UncheckedArray[T]](alloc(n * sizeof(T)))`

Also, a compiler error message is the opposite of a "runtime error".