add helper function to convert chirrtl to firrtl. (#1854) (#1895)

* add convert(chirrtl: cir.Circuit): fir.Circuit to convert chirrtl to firrtl.

* add scaladoc.

* add test.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit 7dd2d7db

)

Co-authored-by: Jiuyang Liu <liu@jiuyang.me>