Fix equality comparison for StringText

This being a container for a source string, while identical StringTexts will  have the same GetHashCode(), it fails for equality testing because a StringText object is compared to the other's source text, which is always false.

The comparison to string behavior is retained.

Author: deviousasti

src/utils/prim-lexing.fs

@@ -52,7 +52,11 @@ type StringText(str: string) =
     member __.String = str
 
     override __.GetHashCode() = str.GetHashCode()
-    override __.Equals(obj: obj) = str.Equals(obj)
+    override __.Equals(obj: obj) = 
+        match obj with
+        | :? StringText as other -> other.String.Equals(str)
+        | :? string as other -> other.Equals(str)
+        | _ -> false        
     override __.ToString() = str
 
     interface ISourceText with