elang/compiler/ast: Introduce |ast::NamedNode| to share code among named node, e.g. |ast::LocalVariable|, |ast::EnumMember| and |ast::NamespaceMember|.
elang/compiler/testing: Move |testing::Formatter| to own file along with introducding namespace |::elang::compiler::testing|.
elang/compiler: Rename |NameResolver| to |NamespaceAnalyzer|.
elang/compiler: Move |Lexer| and |Parser| to "elang/compiler/syntax/".