Add DirectivesMustBeValid schema phase

[maartenvanvliet]

I noticed in the SDL rendering output that a macro directive that did not
specify the on clause would render invalid SDL.

E.g.

    directive :foo do
      arg :baz, :string
    end

Would render in SDL

directive @foo(baz: String) on

So the directive locations are missing making it invalid.

This PR adds a phase to the schema pipeline to check whether
the directive locations are set. I also check whether the locations
themselves are valid.

[binaryseed]

Awesome!

[maartenvanvliet]

Thanks for merging!