Add annotate_snippets for better parsing errors (#477)

Adds nicer errors for Podlang code, using the `annotate_snippets` crate, the same crate used by the Rust compiler to generate contextual errors. This prints a short snippet of the code containing the error within the error message, highlighting the part that needs to be fixed.

It also includes a change to the `load_module` function, changing a `Vec` function argument to a slice.

src/backends/plonky2/mainpod/mod.rs

@@ -1175,7 +1175,7 @@ pub mod tests {
             "#,
             "test",
             &params,
-            vec![],
+            &[],
         )
         .unwrap();
         let batch = module.batch.clone();