Reference:
* https://radu-matei.com/blog/practical-guide-to-wasm-memory/ [practical-guide-to-wasm-memory.pdf](https://playsecurity.org/doc/showDocDetail.jssp?id=6143)
* https://github.com/alec-deason/wasm_plugin/blob/main/guest/src/lib.rs
* https://github.com/reem/rust-memalloc/blob/master/src/lib.rs

**rust-memalloc** implements `reallocate`:
```rust
pub unsafe fn reallocate(ptr: *mut u8, old_size: usize, new_size: usize) -> *mut u8 {
    if old_size > new_size {
        let mut buf = Vec::from_raw_parts(ptr, new_size, old_size);
        buf.shrink_to_fit();

        ptr_from_vec(buf)
    } else if new_size > old_size {
        let additional = new_size - old_size;

        let mut buf = Vec::from_raw_parts(ptr, 0, old_size);
        buf.reserve_exact(additional);

        ptr_from_vec(buf)
    } else {
        ptr
    }
}
```