Postgres Drop() implementation incomplete #626

Describe the Bug
Migrate.Drop() function only drops tables, not custom types.
When a migration creates a custom type a drop followed by an up fails with type already exists.

Steps to Reproduce
Steps to reproduce the behavior:

1. My migration looks like 'CREATE TYPE mytype AS ENUM('a', 'b');'
2. I ran migrate with func(m *migrate.Migrate) {m.Drop();m.Up()}
3. See error: migration failed: type "mytype" already exists in line 0: CREATE TYPE mytype AS ENUM('a', 'b');
   (details: pq: type "mytype" already exists)

Expected Behavior
Migrate.Drop() should also drop custom types so a Migrate.Up() succeeds on a cleaned database.

Migrate Version
4.15.0
Obtained by checking: go.mod

Loaded Source Drivers
file

Loaded Database Drivers
postgres

Go Version
go1.16.8 linux/amd64
Obtained by running: go version