Updates for GHC 8.8
The two main classes of update are related to MonadFail and type alias expansion.
The MonadFail updates introduce explicit MonadFail instances and backward-compatible `fail` implementations under `Monad` for older GHC versions.
The type alias expansion rules changed in GHC 8.8 in a way that breaks the `Simple Lens` idiom; instead, we have to use `Lens'`. Lens started supporting this alias in version 3.8, which was released in 2013.
This change includes necessary submodule updates, as well as the update for the split of what4 into its own repository.
This was an artifact of merging macaw-semmc into macaw. This commit unifies the
two directories. There are some remaining submodules in the old directory:
these are obsolete and will be removed in a separate commit.
This code was mostly architecture independent already, so this commit moves it
to the macaw-semmc module so that it can be shared with the ARM backend. I
still plan to move the main TH module with the SimpleBuilder to macaw
translation, but that requires a few other changes first.
