Try out agoric's bundle-source and import-bundle

As part of endo, agoric is developing [bundle-source](https://github.com/Agoric/agoric-sdk/tree/master/packages/bundle-source) and [import-bundle](https://github.com/Agoric/agoric-sdk/tree/master/packages/import-bundle), which is intended to do a lot of what our Snaps bundle format is doing, but preserves much better debugging (like preserving line numbers).

We are unsure if these tools will work (and expect they have some issues in them!), and so Agoric requests that we try them out, and let them know what issues (if any) we hit.

They recommend we:
- Copy these packages to the `endo` org from the agoric-sdk monorepo
- Try to generalize them for our purposes
- Finalize their interfaces

Start by reviewing the linked agoric-sdk bundle-source and import-bundle packages and comparing them with Snaps' existing bundle format. Try the packages in this repository, record any issues encountered, and use those findings to define generalized interfaces; done means the tools work for the intended use or the incompatibilities are documented.

Written by the indexing model from the issue text.