Bringing some observable sharing to GHCi

[David Feuer]

The :sprint, :print, and :force commands in GHCi fall into infinite
loops when confronted by cyclical data. This bit me hard in
https://phabricator.haskell.org/D4085 because that produces cyclical
TypeReps, which is trouble for (e.g.) the test break011 which tries to
:force a SomeException (which wraps an Exception dictionary, which has
a Typeable constraint). I could try coming up with a fix myself, but
I'm rather curious whether some of the work you (or others) have
already done on observing GHC data graphs could be yanked into GHCi
itself for this purpose. We want

1. To be able to display cyclical data in some sensible way.

> x = "hi" : x
> x `seq` ()
> :print x

should print some useful representation of x.

2. To be able to force cyclical data without looping.

> x = "hi" : x
> :force x

should print a useful representation of x.