Remove memory_id requirement

[lastmjs]

Requiring a memory id is not desirable, as it adds complexity to the creation of stable structures. I just ran into an issue where a dependency of my application was also using stable structures under-the-hood, and it had decided to use memory_ids 0, 1, and 2. My application of course was also using these memory ids. We had a clash, thus I had to figure this out by looking through source code, and eventually I changed my own application.

Not an ideal situation, and these clashes and issues I imagine will get more painful over time.