util: provide IS_ARRAY_ELEMENT, ARRAY_INDEX, ARRAY_INDEX_FLOOR

`IS_ARRAY_ELEMENT(array, ptr)` behaves like `PART_OF_ARRAY()`
except that it checks that `ptr` is aligned to an array-element
boundary.

`ARRAY_INDEX(array, ptr)` returns the index of `ptr`, where
`ptr` is aligned to an array-element boundary.

`ARRAY_INDEX_FLOOR(array, ptr)` returns the index of `ptr`,
where `ptr` does not need to be aligned to an array-element
boundary.

If `CONFIG_ASSERT=y`, then `ARRAY_INDEX()` and
`ARRAY_INDEX_FLOOR()` will trigger an assertion for invalid
`ptr`.

Signed-off-by: Christopher Friedt <cfriedt@fb.com>