Rename to avi.normal-mode
`:` displays `:` on command-line
`:abc` echos `:abc` on the command-line
Eventmap handlers receive a copy of the event
characters typed after `:` echo on the command-line
Remove duplicate `beep` fns
Clarify command-line-mode :else clause
Extract avi.command-line-mode/enter
Use defmulti to dispatch on mode
Extract arg-named