libsyntax: De-`@mut` `SCTable::table`
libsyntax: De-`@mut` `SCTable::mark_memo`
libsyntax: De-`@mut` `SCTable::rename_memo`
libsyntax: De-`@mut` `SCTable`
libsyntax: De-`@mut` `last_span` in the parser
libsyntax: De-`@mut` `buffer_start` and `buffer_end` in the parser
libsyntax: De-`@mut` the obsolete set
libsyntax: De-`@mut` `ExtCtxt::backtrace`
libsyntax: De-`@mut` `BlockInfo::pending_renames`
libsyntax: De-`@mut` `Parser::span`