Add some basic test.

src/error.rs

@@ -0,0 +1,19 @@
+/// Main library error type.
+#[derive(thiserror::Error, Debug)]
+pub enum Error {
+    #[error("invalid shapes in {op}, lhs: {lhs:?}, rhs: {rhs:?}")]
+    BinaryInvalidShape {
+        lhs: Vec<usize>,
+        rhs: Vec<usize>,
+        op: &'static str,
+    },
+
+    #[error("unexpected rank, expected: {expected}, got: {got} ({shape:?})")]
+    UnexpectedNumberOfDims {
+        expected: usize,
+        got: usize,
+        shape: Vec<usize>,
+    },
+}
+
+pub type Result<T> = std::result::Result<T, Error>;