rust: alloc: layout: implement ArrayLayout::size()

Provide a convenience method for ArrayLayout to calculate the size of
the ArrayLayout in bytes.

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>