Transfer specs from verification repo

[ebuchman]

Initially a lot of specs were being written in https://github.com/informalsystems/verification/tree/develop.

Do some or any of those need to be moved in here or have they been replaced? I believe we're in the following state:

• fast sync specs are already in this repo
• light client has a PR in this repo
• fork scenarios has a PR in the verification repo that should maybe be moved here

Do we need anything else from verification repo before it's archived/deleted? Can we close all the PRs there? Can we consolidate the issues and move them here?

[ebuchman]

This was done but one is still in a PR #496