Try splitting up What4.Expr.Builder to improve compile times

[RyanGlScott]

what4's compile times are rather slow, and especially so on GHC 9.0 and earlier. On our CI, GHC 9.0 takes about 13 minutes to build, most of it coming from the What4.Expr.Builder module. The story is even more dire for non-x86 architectures. On Debian's build servers, for instance, building what4 times out on the armhf architecture after compiling What4.Expr.Builder for 150 minutes straight (see here). That is so egregiously slow that I cannot help but wonder if there is some other GHC issue at play, but the point remains: What4.Expr.Builder is uncomfortably slow to build.

One idea for improving the compile times is to split up What4.Expr.Builder module into several smaller modules. While this is not guaranteed to improve compile times, my experiences with GHC suggest that this usually provides a modest boost. If nothing else, it would force us to reconsider if we really need to put as much stuff into What4.Expr.Builder as it currently contains.

It is worth noting that the story is far improved on GHC 9.2, which features a much more efficient pattern-match coverage checker. On GHC 9.2, what4 takes about 3 minutes to build, which is about 10 minutes faster than previous versions of GHC. Indeed, most of What4.Expr.Builder consists of this one instance, which pushes the pattern exhaustiveness checker to its absolute limits. Nevertheless, it still takes 3 minutes to build—perhaps we can still do better.