eval: Add DeleteGlobal

Will be use by edit module soon.

pkg/eval/eval.go

@@ -174,6 +174,19 @@ func (ev *Evaler) ExtendGlobal(ns Nser) {
 	ev.global = CombineNs(ev.global, ns.Ns())
 }
 
+// DeleteGlobal deletes a binding from the global namespace.
+func (ev *Evaler) DeleteGlobal(name string) {
+	ev.mu.Lock()
+	defer ev.mu.Unlock()
+	g := ev.global.clone()
+	for i := range g.infos {
+		if g.infos[i].name == name {
+			g.infos[i].deleted = true
+		}
+	}
+	ev.global = g
+}
+
 // Builtin returns the builtin Ns.
 func (ev *Evaler) Builtin() *Ns {
 	ev.mu.RLock()