Use Cabal API for more robust tests

[RyanGlScott]

In #290, I revamped the test suite to be compatible with cabal new-build. Well, to be more accurate, I forced everyone to use cabal new-build, and with an extremely recent version of cabal-install to boot. This "solution" left a sour taste in my mouth, so I sought a more principled approach, which I believe I've found.

We now use a custom Setup.hs script to detect all of the relevant information we'll need to invoke GHC, such as:

• GHC's path
• The database flags to pass it
• The directory in which singletons.cabal is located (previously, we had to pass this manually to the test suite, in one of the more disgusting hacks I've ever put into production)

All of this information is put into an autogenerated module Build_singletons that the test suite uses in turn. This approach was inspired by the widely used cabal-doctest library, which performs similar tricks to figure out the right flags to pass to doctest (which is essentially a wrapper around GHC). Not unrelatedly, much of the code in Setup.hs is cargo-culted from cabal-doctest itself.

The big win from this patch is that we are no longer tied to a particular build tool—the tests now pass with cabal old-build, cabal new-build, and stack! This should hopefully make life easier when testing locally, testing on Travis, and for others who want to try the tests out.

[goldfirere]

I am unqualified to review this patch, but very very grateful that you've looked into this. If it works, merge!