Refactor Zaplib to operate on instances, rather than exported global functions

[disambiguator]

The idea: A lot of the ergonomics of the framework would be easier if we had instances of zaplib instead of relying on imports.

1. You don’t have to worry about calling initialize/initializeWorker twice, because initializing gives you the object to call callRust on and not at the top level
2. you can move some of the awkward let variables at the global scope in the module into easy to read instance vars. less global vars in general.
3. much easier to find in your codebase where zaplib first gets used
4. you’ll need this anyways if you want multiple wasm binaries
5. a bit easier to unify the api between worker and main thread (for example initializePort would be on the module not on the instance)