At compile-time, verify type definitions during derive for postgres

```sql
CREATE TYPE color AS ENUM ( 'red', 'green', 'blue' );
```

```rust
#[derive(sqlx::Type)]
#[sqlx(rename = "color", rename_all = "lowercase")]
enum Color { Red, Green, Yellow }
```

The above should be a compile-time error. It's currently a subtle bug as it doesn't actually error in any meaningful way apart from not being able to load or store correctly.
