fix(codegen): Fixes codegen on VS2019 (#629)

- [X] Fixes code gen on VS2019
- [X] Fixes schema generator not iterating through all nodes
- [X] Fixes errors with building the actual code gen

Projects/SerializationGenerator/SerializerSyntaxReceiver.cs

@@ -13,9 +13,9 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
  *************************************************************************/
 
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Collections.Immutable;
-using System.Linq;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using SourceGeneration;