Inconsistent behavior on errors starting at GHC 8.0.1 due to callstacks

[glguy]

Now that GHC 8 has callstack printing for uses of error it seems tricky to write doctests that work across multiple versions of GHC.

Example:

### Failure in src/Control/Lens/Traversal.hs:639: expression `[] ^. singular _head'
expected: *** Exception: singular: empty traversal
 but got: *** Exception: singular: empty traversal
          CallStack (from HasCallStack):
            error, called at src/Control/Lens/Traversal.hs:663:46 in main:Control.Lens.Traversal

It might be worth omitting call stack printing somehow for the purposes of writing doctests. This would keep the examples concise and allow them to work more portably.

[soenkehahn]

Note that you can maybe work around this issue with ...: https://github.com/sol/doctest#matching-arbitrary-output.