Dropping treediff dependency.

Re-implementing the diffing algorithm to fix an issue when "kind" of
the node changes (array into an object or vice-versa), #37.

Fix some invalid tests, add more tests.

Cargo.toml

@@ -12,14 +12,13 @@ edition = "2021"
 
 [features]
 default = ["diff"]
-diff = ["treediff"]
+diff = []
 
 [dependencies]
 jsonptr = "0.4.7"
 serde = { version = "1.0.159", features = ["derive"] }
 serde_json = "1.0.95"
 thiserror = "1.0.40"
-treediff = { version = "4.0.2", features = ["with-serde-json"], optional = true }
 utoipa = { version = "4.0", optional = true }
 
 [dev-dependencies]