Fix #207

This puts in place a fix for duplicate registers being added during
expression expansion.  Its not clear whether there are any other places
where this can happen, but none are being triggered on our (largish)
example.

src/cgo-main.rs

@@ -56,7 +56,4 @@ fn main() {
     let trace =
         cgo::compute_trace_from_file(&mut corset, &args.tracefile, args.fail_on_missing).unwrap();
     //
-    for col in trace.ids {
-        println!("COLUMN: {col}");
-    }
 }

src/cgo.rs

@@ -124,7 +124,8 @@ impl Trace {
         let crefs = corset.columns.columns_of(reg_id);
         //
         if crefs.is_empty() {
-            todo!("NO COLUMNS ASSIGNED FOR: {handle}")
+            // This should be unreachable.
+            unreachable!("No column assigned to register ({handle}:{reg_id})")
         } else {
             // I'm assuming every register is mapped to at least one
             // column.

src/transformer/selectors.rs

@@ -23,13 +23,13 @@ fn do_expand_expr(
             // TODO: replace name with exprs hash to 100% ensure bijectivity handle/expression
             // Only insert the computation if a column matching the expression has not already been created
             if cols
-                .insert_column_and_register(
+                .maybe_insert_column_and_register(
                     Column::builder()
                         .handle(new_handle.clone())
                         .kind(Kind::Computed)
                         .build(),
                 )
-                .is_ok()
+                .is_some()
             {
                 validate_computation(new_cs, e, &new_handle);
                 let _ = comps.insert(