Better handling of getting valid bound type.

UpsilonTests/GeneralTests/StringTests.cs

@@ -14,8 +14,8 @@ namespace UpsilonTests.GeneralTests
         public void BasicStringVariable()
         {
             const string input = @"
-string = ""test""
-return string
+str = ""test""
+return str
 ";
             var evaluated = Executor.EvaluateScript<string>(input, Options);
             Assert.Equal("test", evaluated);
@@ -25,8 +25,8 @@ return string
         public void StringIndexable()
         {
             const string input = @"
-string = ""test""
-return string[3]
+str = ""test""
+return str[3]
 ";
             var evaluated = Executor.EvaluateScript<string>(input, Options);
             Assert.Equal("s", evaluated);
@@ -36,8 +36,8 @@ return string[3]
         public void StringAddition()
         {
             const string input = @"
-string = ""test"" + ""123""
-return string
+str = ""test"" + ""123""
+return str
 ";
             var evaluated = Executor.EvaluateScript<string>(input, Options);
             Assert.Equal("test123", evaluated);