Set slot NAME and OWNER from the constructor argument NEWNAME if that
has the from "<OWNER>/<NAME>". Actually, if the new slot FULL-NAME is
set in the constructor arguments use that instead.
This allows writing less to get an gh-repos-repo object suitable for
any api request. Instead of
(gh-repos-repo "repo" :name "<NAME>"
:owner (gh-user "owner" :login "<OWNER>"))
which is to long, simply
(gh-repos-repo "repo" :full-name "<OWNER>/<NAME>")
can be used now.