make output type more accurate

[agarwal]

We currently define type phrase, but the definition isn't really accurate. It assumes a separation of stderr and stdout, but actually these outputs should be interleaved. Also exceptions aren't mentioned. The correct type to represent outputs is probably something like:

type output = [`Stdout of string | `Stderr of string] list * exn option

This captures all stdout and stderr in the order that output occurs. It also includes an exception that may have been raised.

The harder part is to:

• [x] Adjust the script to retain stdout and stderr in the order they are output.
• [x] Robustly capture any exception raised by the evaluated expression, being sure not to conflate that with exceptions raised by the code evaluating the code.
• [x] If possible, also distinguish between the output that is a rendering of the evaluated phrase vs output that is a side effect of evaluating the phrase. In this case, see if something better than string should be used to represent the result. Maybe some type from the Outcometree or Toploop modules.

[Chris00]

There is still the goal of capturing stdout and stderr in the right order while distinguishing them but I do not think it is easy to do—short of modifying the underlying C library but I do not think we want to do that.

[agarwal]

Closing this issue. It is essentially done, short of the last point mentioned by @Chris00 which is not a requirement at this time.