Update static Nix build (#2001)

This is a revamp of our use of
[fully static Haskell executables](NixOS/nixpkgs#43795).
The main changes are:

* We can now use a newer revision of Nixpkgs

  The current revision we're using corresponds to Nixpkgs 20.03

* We can now more easily update Nixpkgs

  The logic in this repository more closely tracks how `static-haskell-nix`
  works, so whenever that repository updates Nixpkgs we can pick up the change
  here, too.

  Note that we cannot currently use Nixpkgs `master` due to
  [this issue](NixOS/nixpkgs#85924), but once that is
  fixed when we can track Nixpkgs `master` more freely.

* We no longer need to keep track of separate Nixpkgs revisions for
  static vs. non-static builds

  The two builds can also share dependencies, too.  Only the top-level
  derivations differ now.

The reason that this doesn't use `static-haskell-nix` directly is because of
[`cabal2nix` evaluation failures](nh2/static-haskell-nix#73),
so this change works by lifting just the minimum logic that we need from
the `static-haskell-nix` repository until we can finally use that
repository (or Nixpkgs directly when it's upstreamed).

dhall/benchmark/deep-nested-large-record/Main.hs

@@ -2,7 +2,7 @@
 module Main (main) where
 
 import Data.Void (Void)
-import Gauge     (defaultMain)
+import Gauge (defaultMain)
 
 import qualified Data.Sequence   as Seq
 import qualified Dhall.Core      as Core

dhall/benchmark/parser/Main.hs

@@ -6,11 +6,10 @@ import Control.Exception (throw)
 import Control.Monad     (forM)
 import Data.Map          (Map, foldrWithKey, singleton, unions)
 import Data.Void         (Void)
-import Gauge             (bench, bgroup, defaultMain, env, nf, nfIO, whnf)
+import Gauge             (bench, bgroup, defaultMain, env, nf, whnf)
 
 import System.Directory
 
-import qualified Codec.Serialise
 import qualified Data.ByteString.Lazy
 import qualified Data.Text            as T
 import qualified Data.Text.IO         as TIO
@@ -50,7 +49,7 @@ benchExprFromText name expr =
 
 benchExprFromBytes
     :: String -> Data.ByteString.Lazy.ByteString -> Gauge.Benchmark
-benchExprFromBytes name bytes = bench name (nf f bytes)
+benchExprFromBytes name bs = bench name (nf f bs)
   where
     f bytes =
         case Dhall.Binary.decodeExpression bytes of

dhall/dhall.cabal

@@ -702,12 +702,11 @@ Benchmark dhall-parser
         dhall                                         ,
         directory                                     ,
         gauge                     >= 0.2.3    && < 0.3,
-        serialise                                     ,
         text                      >= 0.11.1.0 && < 1.3
     Default-Language: Haskell2010
     Other-Extensions:
         TypeApplications
-    ghc-options: -rtsopts
+    ghc-options: -rtsopts -Wall
 
 Benchmark deep-nested-large-record
     Type: exitcode-stdio-1.0
@@ -719,3 +718,4 @@ Benchmark deep-nested-large-record
         dhall                                         ,
         gauge                     >= 0.2.3    && < 0.3
     Default-Language: Haskell2010
+    ghc-options: -Wall