Use ConnectEx for non-blocking connect on windows when available.

Still fallback to blocking connect with WSAConnect when ConnectEx is not
available or applicable, but ConnectEx is preferred and recommended by
Microsoft.

Also make some changes to our use of OVERLAPPED in various places in the
ev code, replacing all uses with JanetOverlapped. This also let's us
avoid reusing internal fields for OVERLAPPED which may or may not be
used in various places.

.github/workflows/test.yml

@@ -56,6 +56,27 @@ jobs:
         shell: cmd
         run: build_win dist
 
+  test-windows-sanitizers:
+    name: Build and test on Windows with sanitizers
+    strategy:
+      matrix:
+        os: [ windows-latest ]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - name: Checkout the repository
+        uses: actions/checkout@master
+      - name: Setup MSVC
+        uses: ilammy/msvc-dev-cmd@v1
+      - name: Build the project
+        shell: cmd
+        run: set SANITIZE=1 & build_win
+      - name: Test the project
+        shell: cmd
+        run: set VERBOSE=1 & build_win test
+      - name: Test installer build
+        shell: cmd
+        run: build_win dist
+
   test-windows-min:
     name: Build and test on Windows Minimal build
     strategy: