Support writes in the SQL adapter

[msullivan]

The trickiness here is in supporting all of the various special edgeqlisms that affect writes:

• Rewrites
• Triggers
• Access policies? (We'd probably want that to work on reads also -- which is doable)
• Cross-table exclusive constraints
• Built-in foreign keys
• required multi pointers

My theory for how to implement this, which is not fully formed, is to turn the DML operations in the SQL queries into not-DML CTEs, and then basically use the edgeql compiler to compile edgeql DML that is rigged up to take those CTEs as input. (Using a generalization of external rvars, probably).
That should then let all our hooks operate.

We'd also probably need to compile in assertions that link uuids actually map to a real object.

There are still a bunch of questions, especially around link tables, but I think this should be workable.