Implement safe_change_column

[rkrage]

There are a subset of "binary-compatible" type conversions in Postgres such that changing a column type does not result in a table rewrite. In order to create a safe method for this, we'll need to have an allowlist of type conversions that are safe from both the perspective of Postgres and Rails.

[rkrage]

At least as of 2013 (LOL), it doesn't seem like Postgres provides a complete matrix of these binary-compatible type conversions: https://postgrespro.com/list/thread-id/1830806

I'll have to do some digging