and documented and adjusted these libraries:
racket/contract/base
racket/contract/exists
racket/contract/parametric (renamed from exists)
racket/contract/region
Use let/ec only when needed (i.e. when raise-blame-error is not used).
Also remove some of the old checking functions from mzlib's object-contract
code that are no longer needed now that we have unified the first-order
checking.