SI-7132 - don't discard Unit type in interpreter

This is my first pull request here, so apologies in advance if I've (likely) screwed something up.

This is to fix a bug in the REPL, apparently introduced in 4a6f54b, in which the `:type` command fully evaluates expressions of type `Unit`, rather than just printing the type name `Unit`.

@paulp I think I'm supposed to ask you to review...