- RawQuery
- CreateTableQuery

Additionally:
- allow custom FK names (limited applicability rn)
- implement GetReverse for AddFK and DropFK
- detect renamed foreign keys
- EqualSignature handles empty models (no columns)

This is a WIP commit.

- Do not implement AppendQuery on Operation level.
This is a leaky abstraction as queries are dialect-specific
and migrate package should not be concerned with how they are constructed.
- AlterTableQuery also is an unnecessary abstraction.
Now pgdialect will just build a simple string-query for each Operation.
- Moved operations.go to migrate/ package and deleted alt/ package.
- Minor clean-ups and documentation.

testChangeColumnType is commented out because the implementation is missing.

Bufixes and improvements:
- pgdialect.Inspector canonicalizes all default expressions (lowercase)
to make sure they are always comparable with the model definition.
- sqlschema.SchemaInspector canonicalizes all default expressions (lowercase)
- pgdialect and sqlschema now support type-equivalence, which prevents unnecessary
migrations like CHAR -> CHARACTER from being created.

Changing PRIMARY KEY and UNIQUE-ness are outside of this commit's scope, because
those constraints can span multiple columns.