Implement progn

Progn evaluates all its arguments, in order, and returns the value of
the last argument after evaluation.
Add multiline support to REPL
Moved some functions to the helpers module
Fix REPL `help` command bug

It didn't show the prompt after the help message...
Implement `set`

Used to set a variable in the current scope.
Add Emacs backup files to .gitignore
No REPL output for
Update interactive demo