Update CFG.Expr pretty printer to show partial expressions.

[brianhuffman]

The predicate part is still omitted, however.

Fixes #337.

The previous behavior would print WithAssertion expressions as

$17 = withAssertion(BVRepr 64, <some assertion>)

while the new version prints them as

$17 = withAssertion(BVRepr 64, partial($16))

Other than the inclusion of the value expression, I don't have any strong preferences about the syntax. Further bike-shedding would be welcome.

[langston-barrett]

Thanks for fixing this!

[robdockins]

If it's easy to do, I'd like to go ahead and also have the predicate be displayed.

[brianhuffman]

If the predicate was just a predicate, then it would be easy. However, the predicate part has type AssertionClassifierTree ext e. I'll have to look and see if a pretty printing function exists.

EDIT: It looks like the AssertionTree module does not provide any pretty printing functions. We should probably add one. I'll make a separate ticket for that.

[robdockins]

Oh, yeah, I forgot about that... OK, if it isn't easy to show the predicate, let's at least put a placeholder in the output so people looking at it know that some information has been omitted.