Make examples programs

[jml]

We had a weird thing where top-level examples dir symlinked into tests, we used the test suite to compile the examples, and then used the doctests to check the examples worked.

At least one side effect of this is we get cabal warnings all the time:

Warning: There were multiple candidates for the Cabal entry " Examples.InputObject
         * /Users/jml/src/graphql-api/Examples/InputObject.hs
         picking: /Users/jml/src/graphql-api/tests/Examples/InputObject.hs

Warning: There were multiple candidates for the Cabal entry " Examples.UnionExample
         * /Users/jml/src/graphql-api/Examples/UnionExample.hs
         picking: /Users/jml/src/graphql-api/tests/Examples/UnionExample.hs

I've made the examples executables, which means they can be edited & run, which I think is a nice property. This lets us remove the symlink & doctest hackery.

In order to preserve the doctest coverage, I've translated the doctests into end-to-end tests. I think this has been a useful exercise, because it already demonstrates that our input object support / documentation is lacking (see comment on DogStuff).

It also paves the way for a ratcheted test coverage thing, which I'm quite keen to try.

[teh]

IIRC I hadn't gotten around to writing the type machinery for deconstructing input objects so this is the way to do it for now :/