Bloodstone Db::tree_names

[parazyd]

Use Case:

First sled version had the Db::tree_names() method. It is useful for tracking the trees in a Db for
external users of the library. In Bloodstone, the management of trees has changed a bit and while
it's possible to patch it easily by iterating over Db.collection_name_mapping, I'm not sure if that is the correct way
since it does not contain the default tree name.

Proposed Change:

Implement Db::tree_names() to behave in the same/similar way to first version of sled.

Who Benefits From The Change(s)?

https://docs.rs/sled-overlay/latest/sled_overlay/