Stop relying on Show for terms

Instead, use prettyShow, a new alias for singleLine (whose name is
questionable anyway).

This will allow adding 'deriving Show' for use in development.

We still create a Show instance based on pretty printing in some tests;
arguably they should switch to this future Show instance, to show all
nitty-gritty details of the inputs triggering failures.
Merge pull request #78 from Blaisorblade/topic/split-show-pretty

Stop relying on Show for terms