enter! should use language-info configure-runtime

As discussed in [this mailing list thread](http://www.mail-archive.com/users%40racket-lang.org/msg22112.html), this is important for `lang`s like Typed Racket. This PR makes `enter!` do what DrRacket already does in this regard.

Note that the first commit is just "Rackety" -- convert some `if`s to `cond`s and `let`s to `define`s.

The second commit makes the substantive change.