S-SQL :ALTER-TABLE: Add suport for adding columns, dropping them, and adding named constraints

The only thing it can't do yet is change a column's datatype, but I don't think that's entirely portable across database systems.

I'm not sure if this is intentional, but the naming scheme for actions was a little inconsistent (There was `:ADD` for adding constraints, `:ADD-COLUMN` for columns and `:DROP-CONSTRAINT` instead of just `:DROP` for constraints), so the new instruction for adding named constraints is called `:ADD-CONSTRAINT` and the old one is kept for backwards compatibility.

This PR also updates the documentation for these additions and adds a minor example.