Migrate example

[ethanfrey]

Builds on #510

I think we no longer have valid migrate implementations in cw-plus, which makes it hard to explain. We should at least provide a stub to serve as documentation and a guide.

My idea was to allow cw-plus to upgrade from, say, v0.8.0 or newer to the current version (but must be <= current version). This migrate should be a no-op, but will allow someone to swap out the code ids to use newer functionality.

Make use of the Semver type from #510 to check for version requirements.

This is mainly meant for example/documentation purposes, go heavy on the comments.

[ethanfrey]

I think hack atom contract in cosmwasm uses migrate

https://github.com/CosmWasm/cosmwasm/blob/main/contracts/hackatom/src/contract.rs#L37-L48

[ethanfrey]

Check that cw2 info.name is the same as current contract.

Check that cw2 info.version is >= "0.8.0" && <= CURRENT VERSION (from env!)