Move comparison benchmarks for containers and hashmap behind CPP

[sjakobi]

Note that configuring cpp-options in the cabal.project is currently
unsupported: haskell/cabal#7872

Closes #333.

[treeowl]

I'm sorry I didn't speak up earlier, but shouldn't these be runtime flags?

[sjakobi]

It would probably have been possible to implement this via some tasty configuration. I don't think it matters much though – I doubt that we'll want to run the containers and hashmap benchmarks very often, and if we do, recompiling the benchmarks is pretty quick.

If this change turns out to cause problems, it should be pretty easy to revert.

[treeowl]

Yeah, you're probably right. We almost always just want to see what a commit changes.