`eacl/write-schema!` needs to check for existing Relations, and detect deletions to warn about orphaned Relationships

[theronic]

Like SpiceDB, EACL should throw when deleting Relation schemas that would orphan relationships.

Also check for existing schema when writing schema to prevent unique datom conflicts

[theronic]

Addressed in #55

Implementation details:

• eacl.datomic.schema/write-schema! now computes deltas between existing schema and new schema using compare-schema
• Before retracting any relation, it calls count-relationships-using-relation to check for orphaned relationships
• If any relationships exist for a relation being deleted, it throws an informative error: "Cannot delete relation X because it is used by N relationships."
• Schema changes are transacted atomically - if orphan check fails, no changes are made