adjust class/c so the expansion is slightly smaller

that is, instead of (class/c [m (->m integer? integer?)] n) turning
into a call to a constructor that has calls to coerce-contract for
the 'm' method, it instead expands the 'n' method contract into an
identifier reference (that isnt' bound to #f, since #f is coercable to
a contract) and the 'm' contract argument just sits there in the expansion