emit warnings for unused vars

internal/engine.cm

@@ -158,6 +158,7 @@ function analyze(src, filename) {
   var line = null
   var col = null
   var has_errors = _ast.errors != null && length(_ast.errors) > 0
+  var folded = null
   if (has_errors) {
     while (_i < length(_ast.errors)) {
       e = _ast.errors[_i]
@@ -176,7 +177,17 @@ function analyze(src, filename) {
     }
     disrupt
   }
-  return fold_mod(_ast)
+  folded = fold_mod(_ast)
+  if (!_no_warn && folded._diagnostics != null && length(folded._diagnostics) > 0) {
+    _i = 0
+    while (_i < length(folded._diagnostics)) {
+      e = folded._diagnostics[_i]
+      os.print(`${filename}:${text(e.line)}:${text(e.col)}: ${e.severity}: ${e.message}\n`)
+      _i = _i + 1
+    }
+  }
+  folded._diagnostics = null
+  return folded
 }
 
 // Lazy-loaded verify_ir module (loaded on first use)