Switch from stdweb to websys for #21.

When I added web support in February, it was easier to get started with
stdweb, since it has the nice cargo web tool. However, stdweb is
unmaintained, winit is deprecating support for it, and the next steps
for web (downloading maps dynamically) have better support for web-sys.

With Alvin's guidance, I got
https://github.com/dabreegster/minimal_websys_winit_glow_demo working
first. This PR cuts A/B Street over too.

I tested abst and the widgetry demo in both native and web. The only
major regression from stdweb is the canvas placement and size. I
attempted some fixes, but at this point, I'll leave it as a smaller
followup instead.

.github/workflows/main.yml

@@ -102,8 +102,8 @@ jobs:
       - uses: hecrj/setup-rust-action@v1
         with:
           rust-version: 1.46.0
-      - name: Install cargo-web
-        run: cargo install cargo-web
+      - name: Install wasm-pack
+        run: curl https://rustwasm.github.io/wasm-pack/installer/init.sh -sSf | sh
       - name: Cache build
         uses: actions/cache@v2
         with:
@@ -112,5 +112,5 @@ jobs:
                 key: wasm-build
       - name: Build game
         working-directory: game
-        run: cargo web check --target wasm32-unknown-unknown --no-default-features --features wasm
+        run: wasm-pack build --dev --target web -- --no-default-features --features wasm
         # TODO For now, just check that the build works. Later, build fully and package.