If a user provides a layout with a big enough size, the allocator
might panic on an unwrap instead of returning an error. The reason is
that, even though the layout size is properly rounded to a power of
two, Layout::from_size_align() will fail if the size overflows isize
when rounded to the layout alignment.

Fix this by handling the error instead of unwrapping it.

Keep an unwrap in the HoleList::deallocate() as the validity of the
layout must be guaranteed by the caller since it is an unsafe
function.