feat: add expression builder API

argus-core/src/lib.rs

@@ -1 +1,14 @@
 pub mod expr;
+
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error("redeclaration of identifier")]
+    IdentifierRedeclaration,
+    #[error("insufficient number of arguments")]
+    IncompleteArgs,
+}
+
+pub type ArgusError = Error;
+pub type ArgusResult<T> = Result<T, Error>;