Potential Unsound Issue when dealing with ZST

**Describe the bug**
Hi, thanks for your time. Our static analyzer finds the PageVec doesn't check for ZST as element.

**To Reproduce**
Please run folloiwng poc:

```rust
use memory_pages::PagedVec;

#[derive(Debug)]
struct A;
fn main() {
let mut vec = PagedVec::::new(0);
vec.reserve(1000);
println!("{:?}", vec.len());
}
```

**Additional context**
N/A

Start at the PagedVec::new and reserve entry points used by the reproduction, and inspect how zero-sized element types are handled before page calculations. Run the provided Rust program to reproduce the divide-by-zero failure. Done means the ZST case no longer divides by zero and the program exits successfully.

Written by the indexing model from the issue text.