Public MigrationList

I am running into a situation where I'd like to have my set of migrations run across multiple (different types) of databases, but in a controlled manner via sqlx. The most straightforward way to do this would be by leveraging the code you've already written permitting `MigrationList` to act as a `impl MigrationSource`. The modification would be very easy:

Before:

```rust
struct MigrationList(Vec);
```

After:

```
pub struct MigrationList(pub Vec);
```

Although a concern might be polluting the public API with internal smart-constructors, I believe that it's exposure wouldn't undermine any semantic operation of the library - it would only provide a simple extra kit for people trying to manually run their migrations.