elang: Introduce |Castable| class.
elang: Utilize |Castable| class for AST nodes.
elang: Rename include guard symbol definitions to prefix with |INCLUDE_|.
elang/compiler: Introduce |ast::Alias| to replace |AliasDef|.
elang/compiler: Move |ast::Namespace::ImportDef| to private.
elang/compiler: Introduce |ast::NamespaceBody|.
elang/compiler: Introduce |Token| printer.
elang/compiler: Introduce |extern| modifier.
elang/compiler: Introduce class declartion parser test.
elang/compiler: WIP Introduce |NameResolver|.