elang/compiler: Change |NameResolver::ResolveLeftMostName()| and |NameResolver::ResolveQualifiedName()| to return |ast::NamespaceMember*| rather than |Maybe<ast::NamespaceMember*>|. Since, value of name is derteministic.
elang/compiler: Introduce |ast::Alias::target()| to keep resolved target.
elang/compiler: Introduce |ast::Class::base_classes()| and |ast::Class::is_fixed()|.
elang: Rename |maybe(t)| to |make_maybe(t)|.
elang/compiler: Sort error list by source code offset.