and documented and adjusted these libraries:
racket/contract/base
racket/contract/exists
racket/contract/parametric (renamed from exists)
racket/contract/region
original commit: 21cbd9ad81eb35dc4d39d6063db25fcb1cc94bbc
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.
original commit: 96db670d8c5453173b9bf92375512fc57cafbfcd