Rewrite most of juvix-mode

[janmasrovira]

Things in this pr:

1. Much cleaner code.
2. If a call to the juvix command crashes, the error will be displayed on a pop-up window.
3. We save the buffer before inserting the top module name to guarantee it works consistently.
4. We use a dedicated buffer to log debug messages.
5. Add necessary sections (such as ;;; Code:) to remove warnings.