The mentioned functions are documented [1].  However, I'll get the
"Unbound variable" error if I try to import them:

scheme@(guile-user)> ,use (rnrs io ports) 
scheme@(guile-user)> string->bytevector
;;; <unknown-location>: warning: possibly unbound variable `string->bytevector'
ERROR: In procedure #<procedure 8ab8bc0 ()>:
ERROR: In procedure module-lookup: Unbound variable: string->bytevector

(I haven't found them via 'grep' too.)

I'm using Guile 2.0.7.

[1] https://gnu.org/software/guile/manual/guile.html#R6RS-Transcoders