- `data` field is removed, instead the allocation layout is changed such that first element of control bytes start exactly at one past last element of data table. So we just use negative index to access data table without touching size field.
- Rewritten `calculate_layout` to remove padding between two tables, its placed either at start or end of allocation if necessary.

fix build failure

doing `ptr as usize as *mut _` does escape from miri analysis
```
ptr: NonNull::new_unchecked(ptr),
     |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a potentially NULL pointer, but expected something that cannot possibly fail to be greater or equal to 1
```
I am not sure if this is correct way to fix it.

- Removed unnecessary padding at the end of `calculate_layout`
- Put back old comments
- Renamed some functions
- Use `#[cfg(feature = "...")]` instead `#[allow(dead_code)]`

- nightly version of calculate_layout was putting padding between two tables in some cases, fixed. Also added debug_assert! to check this.
- use wrapping_sub() instead sub() in data_start() (empty RawTable returns buckets() == 1, crosses allocation bounds)

- Removed unnecessary code from nightly `calculate_layout`
- Added comment for `Bucket::ptr` data member

removed wrong assertation code