Lots of changes:

    - When possible, we use the container infrastructure (sudo: false)
      rather than Google Compute Engine infrastructure (sudo: required).
      Unfortunately, we can't use GCE for the Linux builds, where
      reduced RAM available hoses are GHC build.

    - Switched from using ./Setup and old-style cabal to new-build.
      There are numerous great benefits but the best is that
      .cabal/store can be cached on Travis, leading to huge speedups
      on the build.  Downside is we need to string-and-ceiling-wax
      support for test/haddock/etc.

    - I stopped bootstrapping on every build we do; instead there
      is a separate bootstrap build we do to make sure that that
      is working.  This also speeds up the basic builds since
      we are not building Cabal/cabal-install multiple times.

    - There are some hacks.  The big one is setting CABAL_BUILDDIR
      explicitly; this smooths over quite a few infelicities in
      the current new-build implementation.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>