Move Hedgehog generators out of the library

The Haskell library component currently depends on Hedgehog, even when building only the library. The only reason for this is `src/Language/GraphQL/Draft/Generator.hs`, which is marked in the `exposed-modules` of the library.

Presumably, the reason for this is that both the test suite and the benchmark use that file.

Can we avoid this dependency on Hedgehog by making `src/Language/GraphQL/Draft/Generator.hs` part of the test suite and benchmark directly? It might end up getting compiled twice (once for the tests, once for the benchmarks). In return, it would not get built at all for the library. This would be preferable over pulling in both Hedgehog and `Language.GraphQL.Draft.Generator` into the library, even though both never get used.