Build History for pull/276

Add support for PostgreSQL-style `ALTER COLUMN` syntax PostgreSQL (and standard SQL) uses `ALTER COLUMN` instead of MySQL's `CHANGE COLUMN` / `MODIFY COLUMN` for modifying existing columns. This adds parsing and schema evaluation for the five main sub-commands: - `ALTER COLUMN col TYPE newtype` - `ALTER COLUMN col SET NOT NULL` - `ALTER COLUMN col DROP NOT NULL` - `ALTER COLUMN col SET DEFAULT expr` - `ALTER COLUMN col DROP DEFAULT` These can be combined with other actions in a single `ALTER TABLE` statement, eg.: ``` ALTER TABLE t DROP COLUMN old_col, ALTER COLUMN col TYPE SMALLINT, ALTER COLUMN col SET NOT NULL; ``` Migration generation (inverse actions + SQL fragments) is also supported for all five variants.