Replace `alloc_zeroed` with `alloc` in `BasicMemory::clone` to avoid
unnecessary zero-initialization before copying the existing contents.
The clone path always overwrites the entire allocated region with
`copy_nonoverlapping`, so zeroing first provides no safety benefit but
doubles the amount of memory work for large buffers. The constructor
still uses `alloc_zeroed` to preserve the invariant that freshly created
linear memory starts zeroed.