Convert srfi/61/cond to use modern module style.
Convert srfi/61/cond to use modern module style.

This ensures that `=>` and `else` have the same bindings as the ones in
`racket/private/cond`; otherwise it would not be possible to use SRFI 61
`cond` from a `#lang racket/base` program/module.

Tested under both `#lang racket` and `#lang mzscheme`.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket