lang: Add a for loop statement for iterating over a list

This adds a for statement which is used to iterate over a list with a
body of statements. This is an important data transformation tool which
should be used sparingly, but is important to have.

An import statement inside of a for loop is not currently supported. We
have a simple hack to detect the obvious cases, but more deeply nested
scenarios probably won't be caught, and you'll get an obscure error
message if you try to do this.

This was incredibly challenging to get right, and it's all thanks to Sam
for his brilliance.

Co-authored-by: Samuel Gélineau <gelisam@gmail.com>

lang/funcs/txn/txn.go

@@ -206,8 +206,14 @@ func (obj *opAddEdge) Fn(opapi *opapi) error {
 			args[x] = struct{}{}
 		}
 		if len(args) != len(obj.FE.Args)+len(edge.Args) {
-			// programming error
-			return fmt.Errorf("duplicate arg found")
+			// previously, a programming error
+			// On 24/nov/2024, Sam and I agreed this should be on.
+			// On 11/jan/2025, Sam and I decided to disable this
+			// check, since it was cause duplicates when a nested
+			// StmtFor was having it's list contents add more than
+			// once. It may be helpful to turn this on when
+			// debugging graph transactions not involved StmtFor.
+			//return fmt.Errorf("duplicate arg found: %v -> %v (%v)", obj.F1, obj.F2, obj.FE.Args)
 		}
 		newArgs := []string{}
 		for x := range args {