Integrate misc changes from the ift3065 assignments

* btl/pervasive.typer (List_foldl): New function.
* btl/list.typer: Remove.

* emacs/typer-mode.el (typer-font-lock-keywords): Highlight "keywords".

* src/REPL.ml (arg_batch): New var.
(arg_defs): Add --batch arg.
(main): Obey it.

* src/elexp.ml (elexp_string): Print data constructors more explicitly.

* src/env.ml (env_cell): Remove.

* src/eval.ml (_eval, io_run): Silence compiler warnings.