Better instantiation workflows

[guybedford]

I think we can improve the instantiation story:

• A simpler interface for instantiation with better defaults
• An easier way to provide custom instantiations of WASI APIs

We also already have #413 to unify the sync and async instantiation APIs.

I'd like to propose that for the next major we include improved instantiation as part of the story, to better support use cases such as multiple virtualizations, or creating custom WASI contexts and passing them into the instantiation API.

[guybedford]

This is coming naturally now with the generator work and soon with source phase support.