changing output printing for `

[shriram]

This output seems a bit confusing to people learning Plait:

> (+ `1 `2)
. typecheck failed: Number vs. S-Exp in:
  +
  (quasiquote 1)

Specifically, the quasiquote, which isn't a concept they've really seen at this point: backtick means "s-expression". Could that be rendered as (s-exp 1), with a corresponding s-exp constructor?