Make setting a script variable to nil remove that variable

Upsilon/BaseTypes/ScriptTable/ScriptTable.cs

@@ -27,13 +27,18 @@ namespace Upsilon.BaseTypes.ScriptTable
             {
                 return o;
             }
-            diagnostics.LogError("Cannot find member 's' on Table", span);
+            diagnostics.LogError($"Cannot find member '{s}' on Table", span);
             return new ScriptNull();
         }
 
         public void Set(Diagnostics diagnostics, TextSpan span, ScriptType index, ScriptType value)
         {
             var s = index.ToString();
+            if (value.Type == Type.Nil)
+            {
+                EvaluationScope.Delete(s);
+                return;
+            }
             EvaluationScope.CreateLocal(new VariableSymbol(s, value.Type, false), value);
         }
 

Upsilon/Evaluator/EvaluationScope.cs

@@ -59,6 +59,16 @@ namespace Upsilon.Evaluator
             Variables[symbol.Name] = value;
         }
 
+        public void Delete(VariableSymbol symbol)
+        {
+            Variables.Remove(symbol.Name);
+        }
+
+        public void Delete(string name)
+        {
+            Variables.Remove(name);
+        }
+
 
         public bool TryGet(VariableSymbol symbol, out ScriptType obj)
         {