Replace $letrec with $rec

Replace $letrec with $rec

* module/language/cps.scm ($rec): Replace $letrec with $rec, which is an
  expression, not a term.  This means that the names bound by the letrec
  appear twice: once in the $rec term, and once in the continuation.
  This is not very elegant, but the situation is better than it was
  before.  Adapt all callers.

* doc/ref/compiler.texi (CPS in Guile): Incomplete documentation
  updates.  I'll update these later when the IL settles down.