Add CI to test on i386 (#1147)

The configuration is taken from the bytestring package at
https://github.com/haskell/bytestring/blob/c63dd723/.github/workflows/ci.yml#L186

Drop an unused import and fix a missing include.

Author: meooow25

.github/workflows/i386.yml

@@ -0,0 +1,31 @@
+name: i386 CI
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  i386:
+    runs-on: ubuntu-latest
+    container:
+      image: i386/ubuntu:bionic
+    steps:
+    - name: Install
+      run: |
+        apt-get update -y
+        apt-get install -y autoconf build-essential zlib1g-dev libgmp-dev curl libncurses5 libtinfo5 libncurses5-dev libtinfo-dev
+        curl --proto '=https' --tlsv1.2 -sSf https://get-ghcup.haskell.org | BOOTSTRAP_HASKELL_NONINTERACTIVE=1 BOOTSTRAP_HASKELL_INSTALL_NO_STACK=1 sh
+    - uses: actions/checkout@v1 # This version must stay old enough to remain compatible with the container image
+    - name: Test
+      run: |
+        source ~/.ghcup/env
+        cabal update
+        cabal test all

containers-tests/tests/IntSetValidity.hs

@@ -8,6 +8,8 @@ import Data.List (intercalate)
 import Numeric (showHex)
 import Test.Tasty.QuickCheck (Property, counterexample, property, (.&&.))
 
+#include "containers.h"
+
 {--------------------------------------------------------------------
   Assertions
 --------------------------------------------------------------------}

containers/src/Data/IntSet/Internal.hs

@@ -226,9 +226,6 @@ import Data.Coerce (coerce)
 
 #if __GLASGOW_HASKELL__
 import qualified GHC.Exts
-#  if !(WORD_SIZE_IN_BITS==64)
-import qualified GHC.Int
-#  endif
 import Language.Haskell.TH.Syntax (Lift)
 -- See Note [ Template Haskell Dependencies ]
 import Language.Haskell.TH ()