Try to work on build some more.

crates/core/src/class.rs

@@ -8,3 +8,28 @@ pub struct Class {
     pub path: PathBuf,
     pub checksum: String,
 }
+
+impl Class {
+    pub fn is_updated(&self) -> crate::Result<bool> {
+        // If the path is local and that file has not been updated.
+        Ok(self.checksum == sha256::try_digest(self.path.clone())?)
+    }
+
+    pub fn update(&mut self) -> crate::Result<()> {
+        self.checksum = sha256::try_digest(self.path.clone())?;
+        Ok(())
+    }
+}
+
+// TODO: Make it clearer that this is for general files,
+// not nests.
+impl TryFrom<PathBuf> for Class {
+    type Error = crate::Error;
+
+    fn try_from(value: PathBuf) -> Result<Self, Self::Error> {
+        Ok(Self {
+            path: value.clone(),
+            checksum: sha256::try_digest(value)?,
+        })
+    }
+}