Add support to schema editor for cloning table into schema

[Photonios]

This will not be a documented feature. It has too many ways to shoot yourself in the foot. Consider it an experimental addition that is nicely tucked away.

I want to evolve this over time slowly and use it to build confidence that the approach is solid before committing to supporting it.

What this does

Adds support for cloning a table with the same name into a schema in three steps:

1. Clone just the columns
2. Add the indices & constraints
3. Validate the constraints

They're explicitly three separate steps so the caller can do things in-between them. For example, between step 1 and 2, it can write data to the table quickly without paying the overhead of updating the indices for each row.