script/repl improvements

[agle]

• allow reading script from standard input possibly interactively
• improve error messages for mis-typed commands and pass names
• add self-documentation for script commands
• possible real repl support with a line editor like https://github.com/ocaml-community/ocaml-linenoise
• print success message for each command executed
• need a sensible way to pass repl arguments to transforms

Better integration for cram-style tests

• add a cli option that reads an il file, reads a script from a comment at the top, loads the file and then runs the script. Alternatively: tighter integration with running scripts in the actual IL language. (for this style of test)
• add script command that can run shell script to support test predicates