Further attribute alteration

[rnewman]

#78 implemented the basics of attribute alteration. Still to do:

• Adding a :db/unique attribute. This requires checks to be run against the knowledge base.
• Supporting alteration of :db/index.
• If we wish: adding :db/fulltext.
• If we wish: altering valueType.
• When we support them, fleshing out alteration of :db/noHistory and :db/isComponent.

[rnewman]

For the current Rust implementation of Mentat, we support using :db.alter/_attribute to change:

• Altering cardinality one to many.
• Adding :db.unique.
• Changing :db.index.
• … perhaps others?

We do not support:

• Altering :db/fulltext.
• Altering cardinality many to one.
• Altering :db/valueType.
• Removing :db.unique, or other schema datoms.
• Using plain ol' assert and retract to alter attributes: Datomic added this recently.