- Disable cabal-install's haddock on CI
- Temporarily disable cabal check (`-main-is` in tests)
- Remove this is not a lib note
- Tell to run git update-index --assume-unchanged

    ... when changing to -dev or -monolithic configuration.

    Run --no-assume-unchanged unconditionally, when
    changing to the production build.
- Improve auto-gen note in cabal-install.cabal
- Try to build haddock (with GHC-8.4.3 only)

See #4462 for the gory details.

Main things about this commit:

- New 'monolithic' flag on cabal-install, which combines
  all of the tests into a single binary.  It's not very
  much code, and you don't pay for any of it on a release
  build.  I quite like it.  The one downside is that
  we can't also pull in Cabal test suites this way.

- Env vars got moved into travis-common.sh

- travis-script.sh now runs the cabal-tests tests, because
  we aren't sending enough build product over to do them
  on the second Travis run

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

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

Note: hackage-repo-tool doesn't build with Windows, so that
support is commented out.

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

From Travis:

    Warning: /Users/travis/build/haskell/cabal/cabal.project.local: Unrecognized
    field 'ghc-options' on line 6

This reverts commit 6b7ff819.

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

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>