Allow using Asyncify functions from a shared module

As discussed with @kripken earlier today, currently two asyncified WebAssembly modules can't properly interact with each other, because each gets own asyncify_* methods and a global state.

This means that, when module A calls into a module B, and module B schedules an async operation and starts unwinding, the unwinding ends in the module B, and module A thinks that the call has succeeded and finished.

To fix this, we need to allow sharing Asyncify between different modules by providing a single entry point for the implementation that would be imported by all the others.

As a bonus, this would also reduce code duplication between different modules when code splitting and could allow making the Asyncify API implementation more isolated by using a separate memory and not clashing with real linear memory of other modules.

The issue names no files, tests, or entry points. Start by locating the Asyncify implementation and how separate WebAssembly modules interact; done means shared Asyncify behavior works across module calls without the independent global-state failure described here.