Textual representation of different values at the top level

[fpringle]

Closes #16.

Adds a new DiffError constructor TopLevelNotEqualShow. This is similar to TopLevelNotEqual, except that it includes 2 Text arguments that are the result of calling show on the compared values.

Whereas before we would have

printDiffResult $ diff (Atom 1) (Atom 2)
Both values use constructor Atom but fields don't match
In field 0 (0-indexed):
  Not equal

Now we have:

printDiffResult $ diff (Atom 1) (Atom 2)
Both values use constructor Atom but fields don't match
In field 0 (0-indexed):
  Not equal
  Left value:  1
  Right value: 2

[fpringle]

@ozkutuk there are 2 differences between this approach and yours. The first is simple, I decided to add a new constructor TopLevelNotEqualShow for backwards-compatability. Secondly, the arguments are Texts rather than as. This makes the Show and Eq instances trivial, and is equivalent for our use case (since we're only going to call show on them anyway).

An alternative solution which we discussed would have been this:

TopLevelNotEqual :: (Show a, Eq a) => a -> a -> DiffError a

However this ended up being less elegant, and would mean removing some of the existing instances (like MVar, as you mentioned). With the approach I took, we can use the original constructor (without Show) for those types, and the new one for the rest.

[ozkutuk]

Thanks for the super-fast PR! LGTM, just some typos in the Haddocks. Otherwise, the approach seems solid. I can see how using Text instead of a in the constructor makes for a far simpler implementation.

[fpringle]

@ozkutuk Thanks! I shouldn't write documentation late at night 😝