ssh/key.scm (make-keypair): New procedure

* ssh/key-type.c (_scm_from_ssh_key, guile_ssh_make_keypair): New
  procedures.
* ssh/key-type.h: Update.
* ssh/key.scm (make-keypair): Export.
* doc/api-keys.texi (Keys): Add description of `make-keypair'.
* tests/key.scm ("make-keypair"): New TC.
* NEWS: Update.