refactor(xatu): return structured reports

src/xatu/ast.rs

@@ -16,6 +16,12 @@ pub struct SourceRange {
     pub end: usize,
 }
 
+impl std::fmt::Display for SourceRange {
+    fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(formatter, "{}..{}", self.start, self.end)
+    }
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Node {
     pub range: SourceRange,