Add `Database()` to the public `Collection` interface

[herbygillot]

The db.Collection interface does not include a Database() function, even though it is implemented by most every adapter's collection implementation.

If interacting with a collection through the standard db.Collection interface type, there does not seem to be a way to reference the underlying database. (Perhaps I am mistaken?) sqladapter.Collection would be ideal for use, but it's internal and off-limits 😕

Specifically, I'd like to see this method added as part of this interface.