ssh/session.scm (bytevector->hex-string): New procedure.

* ssh/session.scm (bytevector->hex-string): New procedure.
* examples/echo/client.scm.in (main): Use it.
* examples/sssh.scm.in (main): Use it.
doc/api-sessions.texi (Sessions): Update.

* doc/api-sessions.texi (Sessions): Update description of
  `get-public-key-hash'.  Add documentation for
  `bytevector->hex-string'.  Add examples.