Example databases (#499). r=emily

* Update fixtures to match the current storage schema.

The existing files were a little misleading. This commit moves them into an 'old'
directory, and creates a new 'v1empty.db'.

* Add empty example Toodle database.

* Rename :item/labels to :item/label in Toodle schema.

* Define 'cargo cli' as an alias to run the Mentat CLI.

.cargo/config

@@ -0,0 +1,2 @@
+[alias]
+cli = ["run", "-p", "mentat_cli"]

fixtures/old/README.md

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+Mentat has gone through several pre-release changes that are (deliberately) not
+reflected in current version numbers. These old files are preserved for
+historical reference.