This contains a variety of tweaks needed to make `what4`, `what4-abc`, and
`what4-blt` build warning-free on GHC 9.0:
* GHC's constraint solver now solves constraints in each top-level group
sooner (see
[here](https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.0?version_id=5fcd0a50e0872efb3c38a32db140506da8310d87#the-order-of-th-splices-is-more-important)).
This affects `what4`'s `What4.Expr.App` module, as it separates top-level
groups with a Template Haskell `$(return [])` splice. The previous location
of this splice made it so that the TH-generated instances in that module
(e.g., the `TraversableFC` instance for `App`) were not available to any code
before the splice, resulting in type errors when compiled with GHC 9.0.
I implemented a fairly involved fix of moving each of the affected data
types, as well as their corresponding TH-generated instances, to the top of
the module to ensure that subsequent top-level groups have access to this
code.
* GHC 9.0 implements simplified subsumption (see
[here](https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.0?version_id=5fcd0a50e0872efb3c38a32db140506da8310d87#simplified-subsumption)).
This affects the use of the `freshBoundTermFn` function in `what4`'s
`What4.Protocol.SMTWriter` module, as `freshBoundTermFn`'s type signature
contains a nested `forall`. Fortunately, repairing this code is as simple as
a single eta expansion.
* Raise the upper version bounds on `base` in `what4-abc` and `what4-blt` to
permit building with `base-4.15` (bundled with GHC 9.0).
* Bump the `aig` submodule commit so that it does not emit `-Wstar-is-type`
warnings when built with GHC 9.0, where `-Wall` implies `-Wstar-is-type`.
Bump the `language-sally` submodule commit to allow building with
`base-4.15` (see GaloisInc/language-sally#6).
* Update/imporove Haddock comments and bump copyright years
* Bump copyright dates in LICENSE files
* Add .cabal package lower bounds. These appear to be reasonable, but
I haven't exhaustively tested the various configurations.
* Bump copyright dates in .cabal files
* Start a basic changelog
