Ability to tell if a Database is an in-memory one

[simonw]

Currently the constructor accepts memory=True or memory_name=... and uses those to create a connection, but does not record what those values were:

https://github.com/simonw/sqlite-utils/blob/1260bdc7bfe31c36c272572c6389125f8de6ef71/sqlite_utils/db.py#L307-L349

This makes it hard to tell if a database object is to an in-memory or a file-based database, which is sometimes useful to know.

[simonw]

Maybe just populate db.memory: bool and db.memory_name: Optional[str] for this, then document them.