Convert DROP CONSTRAINT SQL into OpDropMultiColumnConstraint operations

[ryanslade]

Convert DROP CONSTRAINT SQL into an OpDropMultiColumnConstraint.
Because we are unable to infer the columns involved, placeholder migrations are used.

SQL statements like the following are supported:

ALTER TABLE foo DROP CONSTRAINT constraint_foo
ALTER TABLE foo DROP CONSTRAINT IF EXISTS constraint_foo
ALTER TABLE foo DROP CONSTRAINT IF EXISTS constraint_foo RESTRICT

CASCADE is currently not supported and will fall back to raw SQL

Part of #504