/// Allocate memory into the module's linear memory
/// and return the offset to the start of the block.
#[no_mangle]
pub fn alloc(len: usize) -> *mut u8 {
    // create a new mutable buffer with capacity `len`
    let mut buf = Vec::with_capacity(len);
    // take a mutable pointer to the buffer
    let ptr = buf.as_mut_ptr();
    // take ownership of the memory block and
    // ensure the its destructor is not
    // called when the object goes out of scope
    // at the end of the function
    std::mem::forget(buf);
    // return the pointer so the runtime
    // can write data at this offset
    ptr
}

#[no_mangle]
pub unsafe fn dealloc(ptr: *mut u8, size: usize) {
    let data = Vec::from_raw_parts(ptr, size, size);
    std::mem::drop(data);
}