Allow manually specifying description of contramapped Predicate

[Brandon Chinn]

Currently, contramap, with, and qWith are all ways to preprocess the value before passing the value to the next Predicate. But they all update the description with a hardcoded method:

• contramap says in a property: ...
• with says property at Foo.hs:1:1: ...
• qWith says <funcname>: ...

This might be fine in most cases, but it can get pretty weird with things like contains:

ghci> explain (contains $ $(qWith [| abs |]) (gt 5)) [2 :: Int]
"all elements abs: ≤ 5"

it'd be nice to do something like

>>> explain (contains $ withAs "after absolute value " abs (gt 5)) [2 :: Int]
"all elements after absolute value ≤ 5"

Also, related, there's no way to contramap without changing the explanation. (IMO, contramap shouldn't change the message at all, but maybe that's a separate discussion). This could be useful for things like unwrapping newtypes.