Allow unprotected identifiers to be exported from with-contract, plus do some error checking on that list.
Allow unprotected identifiers to be exported from with-contract, plus
do some error checking on that list.

svn: r11651

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket