sometimes-faster fixnum `/` via` quotient`
sometimes-faster fixnum `/` via` quotient`

Try `fxquotient` with a `fx*` check to implement `/` on fixnums.
That's fast enough to be much faster when it works, and only slows
down a more general `/` a little.

mflatt/ChezScheme,mflatt/ChezScheme,mflatt/ChezScheme