feat(testlib): implement custom `ToExpr` instance for `Mismatch` datatype
The `Show` instance for Mismatch was modified to show type level information.
The same thing needs to be done for the `TreeDiff` instance.
Example of the `Expr`:
```
ghci> toExpr (Mismatch @RelEQ (42 :: Int) 100)
Rec "Mismatch (RelEQ)" (fromList [("supplied",App "42" []),("expected",App "100" [])])
```