Generate non-deterministic pointer variables

[stogaru]

Requested feature: API to generate pointers non-deterministically

Use case: Currently, to generate arbitrary pointers we are either using the PointerGenerator API or generating a non-deterministic variable and getting a reference to that. For example we are doing one of the following:

let mut generator = PointerGenerator::<4>::new();
let ptr1: *const u32 = generator.any_in_bounds::<u32>().ptr;

OR

let val: u32 = kani::any();
let ptr: *const u32 = &val;

But both of these do not cover the entire address space that a pointer can take. This is important for functions concerned with pointer arithmetic as they do not care about the values within. So, we can have something like:

let ptr: *const u32 = kani::any_alloc_status()

Right now, one way I can think of generating an allocated ptr that can cover the entire address space in the harness is by doing the following:

 let val: $type = kani::any::<$type>();

// pointer can be of any address
let ptr1 = kani::any::<usize>() as *mut $type;

// write_unaligned requires non-overlapping src and dest
// val's address should not overlap with the object ptr1 is pointing to 
kani::assume((ptr1 as usize) > (&val as *const $type as usize));
kani::assume((ptr1 as usize).checked_add(size_of::<$type>()).is_some());
kani::assume((ptr1 as usize + size_of::<$type>()) < (&val as *const $type as usize));

unsafe { ptr1.write_unaligned(val) }

@feliperodri