Merge main rep master to fork master

Moved all `as_standard_layout`-related tests to the separate module

The old implementation incorrectly handled the strides when the array
was not in standard layout. (It created a `Vec` corresponding to an
array in standard layout but didn't update the strides accordingly.)
The new implementation calls `.to_owned()`, which handles the problem
of efficiently creating an owned array, and then moves all the field
values. Additionally, the `ensure_is_owned` function has been removed
because it's equivalent to `DataMut::ensure_unique`.

These functions were used only in the implementation of
`CowRepr::to_owned`. They may turn out to be useful in the future, but
until there's a clear use case that is not served by `.view()` and
`.into_owned()`, let's remove them.

The old implementation panicked when the variants were different; the
new implementation performs the necessary clone instead of panicking.
The variant of `self` after calling `clone_from_with_ptr` should match
the variant of `other`. If the variants are initially different, the
data from `other` needs to be cloned, and `self` needs to be changed
to that variant.

This is analogous to `impl_owned_array`, `impl_views`, and
`impl_raw_views`.

In the vast majority of cases, `A` will implement `Clone`. However,
`ArrayCow` may still be useful for cases where `A` does not implement
`Clone`.

The compiler should have a slightly easier time making this efficient
because there are fewer type conversions this way.

Refine Cow stuff and as_standard_layout impl

Update fork master

Update master

…iguous-method

- reduced the amount of the boilerplate code in related tests
- added explicit assertion about the size of the source array when
  copying it to the standard layout.
- added documentation string to the new method
- removed redundant import