[gambit-list] Cleaning up error checking in the bit-field code

Marc: Here's a small patch that reorganizes the error checking in some of the bit-field code, to remove it as much as possible from the internal ##whatever routines. I ran make check, but I have no idea whether these routines are actually checked by "make check". Oh, for a real testsuite ... Note that there's still a need for a #macro-nonnegative-exact-int? and ##fail-check-nonnegative-exact-integer. Brad