repl.scm: (begin (define ...) ...)

0) no ht_rebalance message
1) `pair?`, `caar`,... moved to functions;
1a) `(pair? '())` is now #f;
2) `define` is not a macro anymore:
    `secd-compile-top` rewrites it to `secd-bind!`
    otherwise it is allowed only after `begin`;
3) `(begin expr1)` is now compiled to just `expr1`;