* New AIR instructions: ptr_add, ptr_sub, ptr_elem_val, ptr_ptr_elem_val
- See the doc comments for details.
* Sema: implement runtime pointer arithmetic.
* Sema: implement elem_val for many-pointers.
* Sema: support coercion from `*[N:s]T` to `[*]T`.
* Type: isIndexable handles many-pointers.