Decide policy for exporting utilities and helper types.

[lthibault]

As per Matrix discussion, we need to come to a decision on how to handle exported utility types such as mpsc that are orthogonal to Cap'n Proto. Three options come to mind:

1. Consider go-capnproto2 as something approximating a monorepo => export under root-level util/* packages
2. Split utilities into stand-alone repositories, e.g. zenhack/mpsc
3. Create a separate util repository

[zenhack]

Reopening; "resolve" in a commit message triggers closing the issue.