Refactoring streams and fixin various bugs

Src/Data/Myc.Data.Keyword.pas

@@ -45,6 +45,8 @@ type
         class function Intern(const AName: string): IKeyword; static;
         // Gets the name for a given keyword index.
         class function GetName(AIdx: Integer): string; static;
+        // Gets the keyword for a given keyword index.
+        class function GetKeyword(AIdx: Integer): IKeyword; static;
     end;
 
     // Defines a mapping from Keywords to a generic value T
@@ -144,6 +146,11 @@ begin
     FReverseMap.Free;
 end;
 
+class function TKeywordRegistry.GetKeyword(AIdx: Integer): IKeyword;
+begin
+    Result := FReverseMap[AIdx];
+end;
+
 class function TKeywordRegistry.Intern(const AName: string): IKeyword;
 var
     idx: Integer;

Src/Data/Myc.Data.Value.pas

@@ -333,7 +333,12 @@ end;
 function TDataValue.AsScalarRecord: IKeywordMapping<TScalar>;
 begin
     if (FKind <> vkScalarRecord) then
-        raise EInvalidCast.Create('Cannot read value as Record.');
+    begin
+        if (FKind = vkRecord) then
+            raise EInvalidCast.Create('Scalar record expected.')
+        else
+            raise EInvalidCast.Create('Cannot read value as Record.');
+    end;
     Result := IKeywordMapping<TScalar>(FInterface);
 end;