Fix Stacked Borrows violation in alloc_extend

[human-0]

Currently, there is a Stacked Borrows violation in alloc_extend because &mut chunks.current[next_item_index..] calls deref_mut on the Vec. This borrows the entire slice and invalidates the existing references.

use typed_arena::Arena;

fn main() {
    let arena = Arena::new();
    let a = arena.alloc(0);
    arena.alloc_extend([0]);
    *a = 1; // reference invalid
}

[thomcc]

Note that this is for a different miri issue than #54 -- both should be merged.

Ideally this would have a test too.